libvirt: pass Host object into firewall class

Instead of passing in a get_connection callback function,
give the firewall class impls direct access to the new Host
object instead. Make use of the fakelibvirt fixture to
avoid needing to mock out the libvirt APIs directly.

Blueprint: libvirt-driver-class-refactor
Change-Id: I8af9a5a2ba4f87ef221b9276df7d08395e44f021
Merge "libvirt: pass Host object into firewall class"