Add xenbus fallback to Xs_transport_unix_client

Fall back to connecting via xenbus if local socket is not available.

Signed-off-by: John Else <john.else@citrix.com>
Add xenbus fallback to Xs_transport_lwt_unix_client

Fall back to connecting via xenbus if local socket is not available.

Signed-off-by: John Else <john.else@citrix.com>