libvirt: speed up list_instances method

Switch the list_instances method to use the new
list_instance_domains method. This removes the last
user of list_instance_ids, so delete that method.

Blueprint: libvirt-domain-listing-speedup
Change-Id: I101c42799589f01a830d974e48a1fd4b9fc60371
Merge "libvirt: speed up list_instances method"