fixup! CA-423172: Xen uses ~265 pages/vCPU, not 256

```
vcpu,memory_overhead_pages,coeff,vms
1,234,234,9223372036854775807
3,754,251.333,9223372036854775807
7,1854,264.857,9223372036854775807
15,3891,259.4,9223372036854775807
31,8490,273.871,9223372036854775807
63,17248,273.778,1447
[INFO]VM memory_overhead_pages = ... + vcpu * 273.871 =~ ... + vcpu * 274
[WARNING]With 1447 VMs it might be possible to trigger OOM        error
```

Author: edwintorok

ocaml/xapi-idl/memory/memory.ml

@@ -204,7 +204,7 @@ module Memory_model (D : MEMORY_MODEL_DATA) = struct
     static_max_mib +++ xen_max_offset_mib +++ D.shim_mib static_max_mib
 
   let shadow_mib static_max_mib vcpu_count multiplier =
-    let vcpu_pages = 265L *** Int64.of_int vcpu_count in
+    let vcpu_pages = 274L *** Int64.of_int vcpu_count in
     let p2m_map_pages = static_max_mib in
     let shadow_resident_pages = static_max_mib in
     let total_mib =