CA-115293: Revert "CA-109854: Don't include memory overhead when creating a VM."

Not including the overhead means we don't include it in the squeezed
reservation - therefore if

(maxmem < host free mem)

and

((maxmem + overhead) > host free mem)

we end up relying on xenguest to complain that we're out of memory.

This reverts commit b634e38590568e1f5b9b00080193b2d8aa870c1e.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1491 from johnelse/ca-115293

Revert "CA-109854: Don't include memory overhead when creating a VM"