libvirt: convert test_host.py to use FakeLibvirtFixture

Instead of monkey patching the Host class to use fakelibvirt,
just use the FakeLibvirtFixture. Also create an instance of
the Host class in setUp since every test case needs it.

Blueprint: libvirt-driver-class-refactor
Change-Id: I1d737fc22bc26b686790c06362eb7c61b26bd39d
Merge "libvirt: convert test_host.py to use FakeLibvirtFixture"