libvirt: convert LibvirtNonblockingTestCase to avoid DB usage

The LibvirtNonblockingTestCase test case does not do anything DB
related. Convert it to NoDBTestCase to avoid DB setup penalty.
This improves perf from 1.8s to 0.4s

Partial-bug: #1369516
Change-Id: I6ff4f46a2e8c2e436d40dd47134f5ec496ce153f
Merge "libvirt: convert LibvirtNonblockingTestCase to avoid DB usage"