libvirt: convert firewall tests to avoid DB usage

The test_firewall.py test suite had alot of code which created
and accessed database objects. This can all be avoided by making
more use of mock to patch out a handful of object access functions.
The test_cidr_rule_nwfilter_xml was deleted entirely since it was
never actually validating any aspect of the nwfilter execution,
so didn't add any benefits over the other test functions that
run.

Partial-bug: #1369516
Change-Id: I7172edace9e568d961fc1dc9f449cd858551bb0d
Merge "libvirt: convert firewall tests to avoid DB usage"