libvirt: remove 'vif_driver' config parameter

The 'vif_driver' config parameter was deprecated in the
previous cycle, so can be deleted now.

The two places in the test file which were overriding
the 'vif_driver' config parameter were redundant as
mocking already ensured the real VIF driver module
was not exercised.

The FakeVIFDriver class is thus no longer required, and
the FakeIptablesFirewallDriver class was already not
required, so both are deleted.

DocImpact
Related-bug: #1302796
Change-Id: I75519266ffc65df92efd20badfeaf88ac0d66f8a
Merge "libvirt: remove 'vif_driver' config parameter"