qemu: Prefer hugepages over mem source='file'

qemu: Prefer hugepages over mem source='file'

https://bugzilla.redhat.com/show_bug.cgi?id=1214369

Consider the following XML:

  <memoryBacking>
    <hugepages>
      <page size='2048' unit='KiB' nodeset='1'/>
    </hugepages>
    <source type='file'/>
    <access mode='shared'/>
  </memoryBacking>

  <numa>
    <cell id='0' cpus='0-3' memory='512000' unit='KiB'/>
    <cell id='1' cpus='4-7' memory='512000' unit='KiB'/>
  </numa>

The following cmd line is generated:

  -object
  memory-backend-file,id=ram-node0,mem-path=/var/lib/libvirt/qemu/ram,
  share=yes,size=524288000 -numa node,nodeid=0,cpus=0-3,memdev=ram-node0
  -object
  memory-backend-file,id=ram-node1,mem-path=/var/lib/libvirt/qemu/ram,
  share=yes,size=524288000 -numa node,nodeid=1,cpus=4-7,memdev=ram-node1

This is obviously wrong as for node 1 hugepages should have been
used. The hugepages configuration is more specific than <source
type='file'/>.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>