libvirt: speed up list_instance_uuids method

Switch the list_instance_uuids method to use the new
list_instance_domains method.

Two tests related to list_instance_uuids were removed,
since the earlier tests for the list_instance_domains
method now capture those scenarios.

Blueprint: libvirt-domain-listing-speedup
Change-Id: Ifaa6f7f45b713592e4e45226111442811d753803
Merge "libvirt: speed up list_instance_uuids method"