libvirt: convert test_vif.py to avoid DB usage

Remove use of the flavour APIs in the VIF test case avoiding
the need to hit the database. This allows conversion to the
NoDBTestCase which speeds up execution from 3 seconds to 0.3
seconds.

Partial-bug: #1369516
Change-Id: Ib0239b0f943baa1b1d81478b3b0d1e2ce1fb8722
Merge "libvirt: convert test_vif.py to avoid DB usage"