conf: Disallow nonexistent NUMA nodes for hugepages

conf: Disallow nonexistent NUMA nodes for hugepages

As of 136ad4974 it is possible to specify different huge pages per
guest NUMA node. However, there's no check if nodeset specified in
./hugepages/page contains only those guest NUMA nodes that exist.
In other words with current code it is possible to define meaningless
combination:

  <memoryBacking>
    <hugepages>
      <page size='1048576' unit='KiB' nodeset='0,2-3'/>
      <page size='2048' unit='KiB' nodeset='1,4'/>
    </hugepages>
  </memoryBacking>
  <vcpu placement='static'>4</vcpu>
  <cpu>
    <numa>
      <cell id='0' cpus='0' memory='1048576'/>
      <cell id='1' cpus='1' memory='1048576'/>
      <cell id='2' cpus='2' memory='1048576'/>
      <cell id='3' cpus='3' memory='1048576'/>
    </numa>
  </cpu>

Notice the node 4 in <hugepages/>?

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