qemu: Only use memory-backend-file with NUMA if needed

qemu: Only use memory-backend-file with NUMA if needed

If this reminds you of a commit message from around a year ago, it's
41c2aa729f0af084ede95ee9a06219a2dd5fb5df and yes, we're dealing with
"the same thing" again.  Or f309db1f4d51009bad0d32e12efc75530b66836b and
it's similar.

There is a logic in place that if there is no real need for
memory-backend-file, qemuBuildMemoryBackendStr() returns 0.  However
that wasn't the case with hugepage backing.  The reason for that was
that we abused the 'pagesize' variable for storing that information, but
we should rather have a separate one that specifies whether we really
need the new object for hugepage backing.  And that variable should be
set only if this particular NUMA cell needs special treatment WRT
hugepages.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1372153

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>