CA-422187: fix NUMA on XS8 (#6824)

On XS8 we always raise an exception when attempting to claim from a
single node.
We wanted to only use soft affinity when the single node claim succeeded
(which is the correct fix on XS9, where this API is available). However
this meant that we've effectively completely disabled NUMA support on
XS8, without any way to turn it on.

Always use soft affinity when the single-node claim API is unavailable,
this should keep NUMA working on XS8.

On XS9 Xen itself would never raise ENOSYS (it has a `err = errno = 0`
on ENOSYS).

Fixes: fb66dfc ("CA-421847: set vcpu affinity if node claim
succeeded")

----
tested with:
* job 4526247 XS8: numaEnabledSerialStart on build
toolstack-ci-watch/289611 containing fb66dfc: fails as expected
* job 4526243 XS8: numaEnabledSerialStart on build ca-422187/289733
containing 677c539: succeeds as expected:
```(XEN) [  868.165385] 'u' pressed -> dumping numa info (now = 868165350914)
(XEN) [  868.165388] NODE0 start->0 size->17301504 free->1491212
(XEN) [  868.165391] NODE1 start->17301504 size->16777216 free->1238913
(XEN) [  868.165393] CPU0...39 -> NODE0
(XEN) [  868.165395] CPU40...79 -> NODE1
(XEN) [  868.165396] Memory location of each domain:
(XEN) [  868.165400] Domain 0 (total: 1933312):
(XEN) [  868.173626]     Node 0: 872292
(XEN) [  868.173627]     Node 1: 1061020
(XEN) [  868.173633] Domain 7 (total: 14068987):
(XEN) [  868.232568]     Node 0: 14068987
(XEN) [  868.232569]     Node 1: 0
(XEN) [  868.232573] Domain 8 (total: 14068987):
(XEN) [  868.291207]     Node 0: 0
(XEN) [  868.291208]     Node 1: 14068987
```

Author: robhoes

ocaml/xenopsd/xc/domain.ml

@@ -1002,13 +1002,17 @@ let numa_placement domid ~vcpus ~cores ~memory affinity =
       in
       let nr_pages = Int64.div memory 4096L |> Int64.to_int in
       try
+        D.debug "NUMAClaim domid %d: local claim on node %d: %d pages" domid
+          node nr_pages ;
         Xenctrlext.domain_claim_pages xcext domid ~numa_node nr_pages ;
         set_vcpu_affinity cpu_affinity ;
         Some (node, memory)
       with
       | Xenctrlext.Not_available ->
           (* Xen does not provide the interface to claim pages from a single NUMA
              node, ignore the error and continue. *)
+          D.debug "NUMAClaim domid %d: local claim not available" domid ;
+          set_vcpu_affinity cpu_affinity ;
           None
       | Xenctrlext.Unix_error (errno, _) ->
           D.info