libvirt: convert volume usage test case to avoid DB usage

Convert LibvirtVolumeUsageTestCase so that it avoids creating an
instance in the database. This allows it to be switched over to
use NoDBTestCase, improving perf from 1.7s to 0.13s.

Partial-bug: #1369516
Change-Id: If2ab915388c2a1ba0565d4aa8bb66b98db535094
Merge "libvirt: convert volume usage test case to avoid DB usage"