virt: helper for processing NUMA topology configuration

Add helper class / methods to nova/virt/hardware.py to assist
in handling the various NUMA related image & flavor extra spec
properties and calculating host usage.

Blueprint: virt-driver-numa-placement
Change-Id: I46c92166d5252f3bf9867dc08935df8c636d9b7d
Merge "virt: helper for processing NUMA topology configuration"