libvirt: add parsing of NUMA topology in capabilities XML

Introduce classes to nova.virt.libvirt.config for parsing
the NUMA topology information in the capabilities XML
schema.

Blueprint: virt-driver-numa-placement
Change-Id: I4948514cf38268663cdcf4bb31f335a19a7ec9d1
Merge "libvirt: add parsing of NUMA topology in capabilities XML"