The ssh, libssh, libssh2 & unix transports all need to use a UNIX socket
path, and duplicate some of the same logic for error checking. Pull this
out into a separate method to increase code sharing.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>