This change adds a libvirt command to list all instance UUIDs with
hw_machine_type unset in their image metadata. This will be useful to
operators attempting to change the [libvirt]hw_machine_type default in
the future as it allows them to confirm if it is safe to change the
configurable without impacting existing instances.
blueprint: libvirt-default-machine-type
Change-Id: I39909ace97f62e87f326d4d822d4e4c391445765
9a5b07d9fc