libvirt: convert driver test suite to avoid DB usage

The LibvirtDriverTestCase uses the database for creating
instances and accessing flavors, which can easily be mocked
in the tests. This allows it to be converted to NoDBTestCase
which improves perf from 4.6s to 0.8s

Partial-bug: #1369516
Change-Id: I45e9ae6af3d5c83b54ad1287f2fba44dee30338f
libvirt: convert conn test case to avoid DB usage

The LibvirtConnTestCase uses the database for alot of random
stuff. With a bunch more mocking, it can be converted to use
NoDBTestCase which improves perf from 50s to 9s.

Updates the comment in _get_cpu_numa_config_from_instance to
be explicit about which test cases remain to be converted
after this change.

Closes-bug: #1369516
Change-Id: I1b585cba39913ff4609c6d86dcafc07e6c04e2b8
Merge "libvirt: convert driver test suite to avoid DB usage"
Merge "libvirt: convert conn test case to avoid DB usage"