vcpu properties gathered from query-hotpluggable cpus need to be passed
back to qemu. As qemu did not use the node-id property until now and
libvirt forgot to pass it back properly (it was parsed but not passed
around) we did not honor this.

This patch adds node-id to the structures where it was missing and
passes it around as necessary.

The test data was generated with a VM with following config:
    <numa>
      <cell id='0' cpus='0,2,4,6' memory='512000' unit='KiB'/>
      <cell id='1' cpus='1,3,5,7' memory='512000' unit='KiB'/>
    </numa>

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1452053