driver: add some helpers for opening secondary driver connections

driver: add some helpers for opening secondary driver connections

Various parts of libvirt will want to open connections to secondary
drivers. The right URI to use will depend on the context, so rather than
duplicating that logic in various places, use some helper APIs. This
will also make it easier for us to later pre-open/cache connections to
avoid repeated opening & closing the same connectiong during autostart.

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