CA-421847: set vcpu affinity if node claim succeeded (#6794)

Only set affinity when VM successfully claim pages on a single node:
```
xensource.log:2025-12-15T17:04:34.833013+00:00 host-34 xenopsd-xc: [debug||38 |Async.VM.start R:b195b91f07ac|xenops]
Domain.numa_placement.(fun).set_vcpu_affinity: setting vcpu affinity for domain 43:
[40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; \x0A 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; \x0A 74; 75; 76; 77; 78; 79]
xensource.log:2025-12-15T17:04:34.907898+00:00 host-34 xenopsd-xc: [debug||74 |Async.VM.start R:d4600f3c365b|xenops]
Domain.numa_placement.(fun).set_vcpu_affinity: setting vcpu affinity for domain 44:
[0; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; \x0A 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; \x0A 37; 38; 39]
```
the corresponding nodes the memory was allocated, only VMs that claimed
successfully the node have the vcpu affinity assigned:
```
# xl debug-keys u && xl dmesg | tail -n 30
(XEN) [21080.047304] d43 (total: 5767675):
(XEN) [21080.047305]     Node 0: 1
(XEN) [21080.047307]     Node 1: 5767678
(XEN) [21080.047309] d44 (total: 5767675):
(XEN) [21080.047311]     Node 0: 5767679
(XEN) [21080.047312]     Node 1: 0
(XEN) [21080.047315] d45 (total: 5014779):
(XEN) [21080.047317]     Node 0: 2637953
(XEN) [21080.047318]     Node 1: 2376830
```
and the xl vcpu-list output:
```
# xl vcpu-list
Name                                ID  VCPU   CPU State   Time(s) Affinity (Hard / Soft)
ws19-x64-clone1                     43     0   42   -b-      14.8  all / 40-79
ws19-x64-clone1                     43     1   53   -b-      14.3  all / 40-79
ws19-x64-clone1                     43     2   57   -b-      19.1  all / 40-79
ws19-x64-clone1                     43     3   59   -b-      11.5  all / 40-79
ws19-x64                            44     0    6   -b-      18.0  all / 0-39
ws19-x64                            44     1   12   -b-      17.5  all / 0-39
ws19-x64                            44     2   14   -b-      11.9  all / 0-39
ws19-x64                            44     3    3   -b-      11.4  all / 0-39
spread-ws19-x64                     45     0   21   -b-      17.9  all / all
spread-ws19-x64                     45     1    1   -b-      12.9  all / all
spread-ws19-x64                     45     2    5   -b-      15.7  all / all
spread-ws19-x64                     45     3   18   -b-      15.3  all / all
```

Author: edwintorok

ocaml/xenopsd/xc/domain.ml

@@ -967,17 +967,26 @@ let numa_placement domid ~vcpus ~cores ~memory affinity =
             Array.map2 NUMAResource.min_memory (Array.of_seq nodes) a
       in
       numa_resources := Some nodea ;
-      let memory_plan =
+      let cpu_affinity, memory_plan =
         match Softaffinity.plan ~vm host nodea with
         | None ->
             D.debug "NUMA-aware placement failed for domid %d" domid ;
-            []
+            (None, [])
         | Some (cpu_affinity, mem_plan) ->
+            (Some cpu_affinity, mem_plan)
+      in
+      let set_vcpu_affinity = function
+        | None ->
+            D.debug "%s: not setting vcpu affinity for domain %d" __FUNCTION__
+              domid
+        | Some cpu_affinity ->
+            D.debug "%s: setting vcpu affinity for domain %d: %s" __FUNCTION__
+              domid
+              (Fmt.to_to_string CPUSet.pp_dump cpu_affinity) ;
             let cpus = CPUSet.to_mask cpu_affinity in
             for i = 0 to vcpus - 1 do
               set_affinity affinity xcext domid i cpus
-            done ;
-            mem_plan
+            done
       in
       (* Xen only allows a single node when using memory claims, or none at all. *)
       let* numa_node, node =
@@ -994,6 +1003,7 @@ let numa_placement domid ~vcpus ~cores ~memory affinity =
       let nr_pages = Int64.div memory 4096L |> Int64.to_int in
       try
         Xenctrlext.domain_claim_pages xcext domid ~numa_node nr_pages ;
+        set_vcpu_affinity cpu_affinity ;
         Some (node, memory)
       with
       | Xenctrlext.Not_available ->