Currently the virDrvConnectOpen method is supposed to handle both
opening an explicit URI and auto-probing a driver if no URI is
given. Introduce a dedicated virDrvConnectURIProbe method to enable the
probing functionality to be split from the driver opening functionality.

It is still possible for NULL to be passed to the virDrvConnectOpen
method after this change, because the remote driver needs special
handling to enable probing of the URI against a remote libvirtd daemon.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>