numa_conf: Introduce virDomainNumaGetMaxCPUID

numa_conf: Introduce virDomainNumaGetMaxCPUID

This function should return the greatest CPU number set in
/domain/cpu/numa/cell/@cpus. The idea is that we should compare
the returned value against /domain/vcpu value. Yes, there exist
users who think the following is a good idea:

  <vcpu placement='static'>4</vcpu>
  <cpu mode='host-model'>
    <model fallback='allow'/>
    <numa>
      <cell id='0' cpus='0-1' memory='1048576' unit='KiB'/>
      <cell id='1' cpus='9-10' memory='2097152' unit='KiB'/>
    </numa>
  </cpu>

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