Moves trusted filter unit tests into own file

Breaks out trusted filter unit tests into their own file. In the
process, we remove all DB accesses from the unit tests and uncruft the
unit tests with mock.

Change-Id: Ic895b06a9f6a224f153f12ca5ab54dc8be931f13
Reduce the complexity of _get_guest_config()

Running flake8 to check for cyclomatic complexity showed that the
libvirt driver's _get_guest_config() method was the worst offender,
with a complexity of 67. This commit simply refactors this method,
splitting off nested cases into their own methods, and reduces the
complextity to 40. No functional changes are included.

Change-Id: Ic27e091f922118edb48b233fb31c702d9528bb75
Merge "Reduce the complexity of _get_guest_config()"
Merge "Moves trusted filter unit tests into own file"