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
41c2aa729f and yes, we're dealing with
"the same thing" again.  Or f309db1f4d 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>

tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args

@@ -10,12 +10,10 @@ QEMU_AUDIO_DRV=none \
 -M pc \
 -m 1024 \
 -smp 2,sockets=2,cores=1,threads=1 \
--object memory-backend-file,id=ram-node0,prealloc=yes,\
-mem-path=/dev/hugepages2M/libvirt/qemu,size=268435456 \
--numa node,nodeid=0,cpus=0,memdev=ram-node0 \
--object memory-backend-file,id=ram-node1,prealloc=yes,\
-mem-path=/dev/hugepages2M/libvirt/qemu,size=805306368 \
--numa node,nodeid=1,cpus=1,memdev=ram-node1 \
+-mem-prealloc \
+-mem-path /dev/hugepages2M/libvirt/qemu \
+-numa node,nodeid=0,cpus=0,mem=256 \
+-numa node,nodeid=1,cpus=1,mem=768 \
 -uuid ef1bdff4-27f3-4e85-a807-5fb4d58463cc \
 -nographic \
 -nodefaults \