libvirt: convert volume snapshot test case to avoid DB usage

Convert the LibvirtVolumeSnapshotTestCase so that it avoids
creating instance objects in the database. This allows it to
be converted to use NoDBTestCase improving perf from 4.2s
to 2.3s

Partial-bug: #1369516
Change-Id: Idf4d29a0df2280c78fe879fa09632c666ff04436
Merge "libvirt: convert volume snapshot test case to avoid DB usage"