Libvirt driver imagebackend refactor

This blueprint proposes cleaning up libvirt driver's imagebackend code and it's callers in the driver module. Specifically the intention is to eliminate imagebackend.Image.cache() and replace it with 2 separate methods, create_from_func() and create_from_image(), to be implemented separately by each backend. cache() currently performs several loosely related functions which are assumed by the various callers in the driver module. This has proved to be a source of bugs as they interact in unexpected ways. Consequently, create_from_image() and create_from_func() will be purely for creating disks. The various callers in the driver module will be updated to explicitly call the backend methods they require.