qemuBuildMemoryBackendStr: Fix hugepages lookup process

qemuBuildMemoryBackendStr: Fix hugepages lookup process

https://bugzilla.redhat.com/show_bug.cgi?id=1196644

This function constructs the backend (host facing) part of the
memory device.  At the beginning, the configured hugepages are
searched to find the best match for given guest NUMA node.
Configured hugepages can have a @nodeset attribute to specify on
which guest NUMA nodes should be the hugepages backing used.
There is, however, one 'corner case'. Users may just tell 'use
hugepages to back all the nodes'. In other words:

  <memoryBacking>
    <hugepages/>
  </memoryBacking>

  <cpu>
    <numa>
      <cell id='0' cpus='0-1' memory='1024000' unit='KiB'/>
    </numa>
  </cpu>

Our code fails in this case. Well, since there's no @nodeset (nor
any <page/> child element to <hugepages/>) we fail to lookup the
default hugepage size to use.

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