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

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

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

The hypervisor drivers can now directly open a nodedev 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>