libvirt: introduce a fixture for mocking out libvirt connections

The various libvirt test suites all have different ways of mocking
out libvirt connections. Some use fakelibvirt, others create their
own lame re-implementation of fakelibvirt, others mock random
methods. To improve sanity, introduce a standard fixture to be used
by all libvirt tests for mocking libvirt. This will ensure that the
libvirt.openAuth method always returns an instance of fakelibvirt
virConnect object. This should allow for a number of the fakelibvirt
reimplementations to be eliminated eventually.

Blueprint: libvirt-driver-class-refactor
Change-Id: I2a61dba5a273455d6b3c15e9cae0f3bff7f641fa
Merge "libvirt: introduce a fixture for mocking out libvirt connections"