libvirt: add support for guest NUMA topology in XML config

Add a libvirt config class for dealing with guest NUMA
topology settings under the <cpu> part of the schema.

Blueprint: virt-driver-numa-placement
Change-Id: Ia928fb5b91e8dbf5bbaf255e9ace2b84fe49b187
Merge "libvirt: add support for guest NUMA topology in XML config"