CA-423173: XAPI underestimates low memory emergency pool size (#6855)

The only Xen command-line related to this is `low_mem_virq_limit`, which
is 64MiB.

A new quicktest has shown that we are sometimes off by ~10MiB (between
`Host.compute_free_memory` and actual free memory as measured by a call
to Xenctrl physinfo) or more, and get failures booting VMs even after
`assert_can_boot_here` said yes. Sometimes the error messages can be
quite ugly, internal xenguest/xenopsd errors, instead of
HOST_NOT_ENOUGH_FREE_MEMORY.

After this change (together with
#6854) the new quicktest
doesn't fail anymore.

PR to feature branch because this will need testing together with all
the other NUMA changes, it may expose latent bugs elsewhere.


The new testcase will get its own PR because it is quite large.

Author: edwintorok

ocaml/squeezed/src/squeeze_xen.ml

@@ -61,7 +61,8 @@ let ( -* ) = Int64.sub
 let mib = 1024L
 
 (** Same as xen commandline *)
-let low_mem_emergency_pool = 1L ** mib
+(** CA-423173: this is `low_mem_virq_limit` Xen, default 64MiB *)
+let low_mem_emergency_pool = 64L ** mib
 
 (** Return the extra amount we always add onto maxmem *)
 let xen_max_offset_kib domain_type =