libvirt: Rewrite _test_finish_migration
libvirt: Rewrite _test_finish_migration

Rewrite the finish_migration tests in anticipation of a major update.
Apart from switching from mox to mock, the principal change is that
we're now mocking at the backend boundary rather than the
_create_image boundary, which means we can test what we do to
individual disks. In the updated finish_migration we no longer use
_create_image, so making this change in advance allows us to see the
functional changes more clearly.

Implements: bp/libvirt-imagebackend-refactor
Change-Id: I439a5cfb054fa05ce156f7bf1d24cc8794d561e1

mikalstill/nova,mikalstill/nova,Juniper/nova,vmturbo/nova,phenoxim/nova,rahulunair/nova,rajalokan/nova,gooddata/openstack-nova,klmitch/nova,klmitch/nova,hanlind/nova,openstack/nova,jianghuaw/nova,jianghuaw/nova,rahulunair/nova,gooddata/openstack-nova,rajalokan/nova,mikalstill/nova,hanlind/nova,rajalokan/nova,openstack/nova,Juniper/nova,vmturbo/nova,hanlind/nova,jianghuaw/nova,jianghuaw/nova,openstack/nova,gooddata/openstack-nova,rajalokan/nova,vmturbo/nova,mahak/nova,vmturbo/nova,mahak/nova,Juniper/nova,mahak/nova,rahulunair/nova,klmitch/nova,phenoxim/nova,klmitch/nova,Juniper/nova,gooddata/openstack-nova