Make libvirt use Flavor object instead of using VirtAPI

This makes the libvirt driver use the Flavor object instead of
getting that information through the VirtAPI/conductor call.

Related to blueprint compute-manager-objects

Change-Id: I484c98068a46c9780d82c886779eb58e783f9b6f
Merge "Make libvirt use Flavor object instead of using VirtAPI"