libvirt: convert CacheConcurrencyTestCase to avoid DB usage

The CacheConcurrencyTestCase test case does not do anything DB
related. Convert it to NoDBTestCase to avoid DB setup penalty.
This improves perf from 1.7s to 0.2s

Partial-bug: #1369516
Change-Id: I8ef124e9f5d86a990722019978c725c955ffb1ea
Merge "libvirt: convert CacheConcurrencyTestCase to avoid DB usage"