CP-309998: ignore small amount of pages in other nodes (#6884)

This is an optimisation for the calculation of the number of nodes until
we fix all the small xen internal allocations for the VM outside the
main guest's memory. This synchronises the behaviour in the RRD
VM.numa_nodes calculation.

    rrdp-squeezed currently has a threshold of >4096 to count the number
    of nodes. This is to ignore small number of internal data structures
    that xen or other kernel devices may sometimes allocate for the VM
    outside the node where the VM's main memory is allocated.

    This is a temporary fix until we account in CP-311303 about these
small number of pages that sometimes appear out of the VM's main node.
In experiments, it's usually a single-digit number like 1. The maximum
    number observed was around ~2200 pages.

    Without this fix, the VM.numa_nodes calculation is different of the
one returned by rrdp-squeezed, and VM.numa_nodes is over-sensitive to
    these small number of pages in other nodes.

Author: edwintorok

ocaml/libs/xenctrl-ext/xenctrlext.ml

@@ -165,7 +165,7 @@ module DomainNuma = struct
       let nodes =
         Array.fold_left
           (fun n pages ->
-            if pages > 0L then
+            if pages > 4096L then
               n + 1
             else
               n