Ensure there is only one instance of LibvirtDriver

The HostState() class in libvirt creates a second instance of
the LibvirtDriver() class, which cause duplicate initialization
of various things. This is pointless, since it was a method on
the original LibvirtDriver() which created the HostState()
instance, and thus it could easily pass in the original object.

Blueprint: compute-driver-events
Change-Id: I0734bfcc1b014406ffe64ef7abb14a533998dea2
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Ensure there is only one instance of LibvirtDriver"