libvirt: convert utils test case to avoid DB usage

The LibvirtUtilsTestCase test case has no need to access the
database. It is also located in the wrong file, as mosts of
the tests for nova.virt.libvirt.utils are in test_utils.py
instead of test_driver.py. Move all the tests into test_utils.py
which already uses NoDBTestCase. This improve performance from
2s to ~0.2s

Partial-bug: #1369516
Change-Id: Id5db3146c524d0208266f754493226856389a722
Merge "libvirt: convert utils test case to avoid DB usage"