This patch adds the 'socket' value to the allowed PCI NUMA affinity
policies, both to the 'hw:pci_numa_affinity_policy' flavor extra spec,
and the 'hw_pci_numa_affinity_policy' image property.
For now the new value is a no-op and remains undocumented. It will be
wired-in in a subsequent patch.
Implements: blueprint pci-socket-affinity
Change-Id: I0680d4e21f3e317ac702b55afef4c87e8acbfc3a