tests: add RTAS command in the protocol

Add a first test to validate the protocol:

- rtas/get-time-of-day compares the time
  from the guest with the time from the host.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
