libvirt: split firewall tests out into test_firewall.py

The test_driver.py test module had a couple of classes which
were testing functionality from firewall.py. Since test_driver.py
is a really huge file, it is better for the firewall class tests
to live in a separate test_firewall.py module

Related-bug: #1369516
Change-Id: Ibb347780b7b79521ce87b08023771381726b0d53
Merge "libvirt: split firewall tests out into test_firewall.py"