libvirt: remove VIF driver classes deprecated in Icehouse

In Icehouse the old VIF driver classes were set as deprecated
to be removed in Juno. Now they can be finally deleted.

Related-bug: #1302796
Change-Id: I7be735062e6cc6aa54dec1bf43d3cce1ac544988
Fix doc build errors in models.py

The docs build has errors like the following:
nova/nova/db/sqlalchemy/models.py:docstring of
nova.db.sqlalchemy.models.relationship:517: ERROR: Unknown interpreted
text role "paramref".

To fix this remove the import for relationship and refer to it
instead using orm.relationship.

Change-Id: I2e0a08438174f0fe0d7ea732670971e65811a700
Merge "Fix doc build errors in models.py"
Merge "libvirt: remove VIF driver classes deprecated in Icehouse"