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

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

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

The hypervisor drivers can now directly open a secret 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>