CA-311475: do not change a domain's memory allocation while it is being… (#6890)

… built

When a domain takes a long time to be built (e.g. >1TiB) then squeezed
might run and attempt to change maxmem, causing the domain build to fail
to complete.

```
2026-02-04T13:59:12.915844+00:00 orca squeezed: [debug||9 ||squeeze_xen] Xenctrl.domain_setmaxmem domid=717 max=6370254848 (was=0)
2026-02-04T13:59:22.878301+00:00 orca squeezed: [debug||3 ||squeeze_xen] Xenctrl.domain_setmaxmem domid=717 max=2075287552 (was=6370254848)
```

Squeezed shouldn't change the maxmem setting on domains that have never
been run (other than to initialize it if 0). In fact another module in
Squeezed had code to detect whether a domain has ever been run, which
has been replaced with checking whether it has an active balloon driver
(if it hasn't reported a balloon driver it is still not very safe to
change it too early).

But that check missed one place that was still setting maxmem, ignoring
the balloon driver's presence. Fix this (hopefully last!) place: if
there is no balloon driver and we attempt to decrease maxmem then just
log a message instead.

Fixes: 9819bdb ("CA-32810: prevent the memory ballooning daemon
capping a domain's memory usage before it has written feature-balloon.")

Author: edwintorok

ocaml/squeezed/src/squeeze_xen.ml

@@ -794,7 +794,8 @@ let make_host ~verbose ~xc =
     (fun domain ->
       let domid = domain.Squeeze.domid
       and target_kib = domain.Squeeze.target_kib in
-      if target_kib < Domain.get_maxmem xc domid then
+      if target_kib < Domain.get_maxmem xc domid && domain.Squeeze.can_balloon
+      then
         Domain.set_maxmem_noexn xc domid target_kib
     )
     domains ;
@@ -828,16 +829,14 @@ let execute_action ~xc action =
           "Not setting target for domid: %d since no feature-balloon. Setting \
            maxmem to %Ld"
           domid target_kib
-    ) else (
-      if can_balloon then
-        Domain.set_target_noexn cnx domid target_kib
-      else
-        debug
-          "Not setting target for domid: %d since no feature-balloon. Setting \
-           maxmem to %Ld"
-          domid target_kib ;
+    ) else if can_balloon then begin
+      Domain.set_target_noexn cnx domid target_kib ;
       Domain.set_maxmem_noexn cnx domid target_kib
-    )
+    end else
+      debug
+        "Not setting target for domid: %d since no feature-balloon. Not \
+         setting maxmem to %Ld"
+        domid target_kib
   with e ->
     debug "Failed to reset balloon target (domid: %d) (target: %Ld): %s"
       action.Squeeze.action_domid action.Squeeze.new_target_kib