Rename 'connection' to 'driver' in libvirt HostState

The HostState class in the libvirt driver still uses the old
'connection' terminology instead of the preferred 'driver'.

Blueprint: compute-driver-events
Change-Id: I942237c329abbb0b857a5363ce71bd4b9377470d
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Rename 'connection' to 'driver' in libvirt HostState"