interface: allow opening with interface:///system and interface:///session URIs

interface: allow opening with interface:///system and interface:///session URIs

Allow the possibility of opening a connection to only the interface
driver, by defining interface:///system and interface:///session URIs
and registering a fake hypervisor driver that supports them.

The hypervisor drivers can now directly open a interface driver
connection at time of need, instead of having to pass around a
virConnectPtr through many functions. This will facilitate the later
change to support separate daemons for each driver.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>