CA-423574: avoid losing stacktraces in Xenopsd (#6891)

Was trying to debug a Xenctrl exception in xenopsd, but all I was seeing
was the exception, not the location it was raised.
With these 2 changes I was able to see the full stacktrace.

Here is how a working stacktrace looks like:
```
 Raised Server_error(INTERNAL_ERROR, [ xenopsd internal error: Domain.Domain_build_pre_failed("Calling 'NUMA placement' failed: Xenctrlext.Unix_error(12, \\"Error when trying to claim memory pages\\")")
 1/60 xenopsd-xc Raised at file ocaml/xenopsd/xc/domain.ml, line 1159
 2/60 xenopsd-xc Called from file ocaml/xenopsd/xc/domain.ml, line 1210
 3/60 xenopsd-xc Called from file ocaml/xenopsd/xc/domain.ml, line 1441
 4/60 xenopsd-xc Called from file ocaml/xenopsd/xc/xenops_server_xen.ml, line 2589
 5/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 6/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 7/60 xenopsd-xc Called from file ocaml/xenopsd/xc/xenops_server_xen.ml, line 2622
 8/60 xenopsd-xc Called from file ocaml/xenopsd/xc/xenops_server_xen.ml, line 2653
 9/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 10/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 11/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 103
 12/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 13/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 103
 14/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 112
 15/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2575
 16/60 xenopsd-xc Called from file list.ml, line 121
 17/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2568
 18/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2727
 19/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 103
 20/60 xenopsd-xc Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 21/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 103
 22/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 112
 23/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2575
 24/60 xenopsd-xc Called from file list.ml, line 121
 25/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2568
 26/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 2727
 27/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 103
 28/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_task.ml, line 112
 29/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 3428
 30/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 3438
 31/60 xenopsd-xc Called from file ocaml/xenopsd/lib/xenops_server.ml, line 3459
 32/60 xenopsd-xc Called from file ocaml/xapi-idl/lib/task_server.ml, line 192
 33/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3522
 34/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 35/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3921
 36/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3531
 37/60 xapi Called from file lib/backtrace.ml, line 210
 38/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3537
 39/60 xapi Called from file ocaml/xapi/context.ml, line 565
 40/60 xapi Called from file ocaml/xapi/context.ml, line 572
 41/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3928
 42/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3531
 43/60 xapi Called from file ocaml/xapi/xapi_xenops.ml, line 3640
 44/60 xapi Called from file ocaml/xapi/context.ml, line 565
 45/60 xapi Called from file ocaml/xapi/context.ml, line 572
 46/60 xapi Called from file ocaml/xapi/xapi_vm.ml, line 347
 47/60 xapi Called from file ocaml/xapi/message_forwarding.ml, line 141
 48/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 49/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 50/60 xapi Called from file ocaml/xapi/message_forwarding.ml, line 1990
 51/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 52/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 53/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 24
 54/60 xapi Called from file ocaml/libs/xapi-stdext/lib/xapi-stdext-pervasives/pervasiveext.ml, line 39
 55/60 xapi Called from file ocaml/xapi/message_forwarding.ml, line 1974
 56/60 xapi Called from file ocaml/xapi/rbac.ml, line 228
 57/60 xapi Called from file ocaml/xapi/rbac.ml, line 238
 58/60 xapi Called from file ocaml/xapi/server_helpers.ml, line 78
 59/60 xapi Called from file ocaml/xapi/server_helpers.ml, line 97
 60/60 xapi Called from file ocaml/libs/log/debug.ml, line 270

```

Author: edwintorok

ocaml/xenopsd/lib/xenops_task.ml

@@ -106,9 +106,10 @@ let with_tracing ~name ~task f =
         result
       with exn ->
         let backtrace = Printexc.get_raw_backtrace () in
+        Backtrace.is_important exn ;
         let error = (exn, backtrace) in
         let _ : (Span.t option, exn) result = Tracer.finish span ~error in
-        raise exn
+        Printexc.raise_with_backtrace exn backtrace
     )
   | Error e ->
       D.warn "Failed to start tracing: %s" (Printexc.to_string e) ;

ocaml/xenopsd/xc/xenops_server_xen.ml

@@ -658,7 +658,11 @@ module Mem = struct
         ~default:(min, ("none", min))
         (reserve_memory_range dbg min max)
     in
-    try f amount id with e -> delete_reservation dbg id ; raise e
+    try f amount id
+    with e ->
+      let bt = Printexc.get_raw_backtrace () in
+      delete_reservation dbg id ;
+      Printexc.raise_with_backtrace e bt
 
   (** Transfer this 'reservation' to the given domain id *)
   let transfer_reservation_to_domain_exn dbg domid (reservation_id, amount) =