arm-autonomy/xenguest-manager: Make stop by default a blocking action

Added -w parameter to xl shutdown to force it not to return until
the action is completed, and modified logging to reflect this

Stop operation can be configured to return immediately using
--nowait parameter, or to force destroy the guest if the graceful
shutdown fails using --kill. Both are now documented in usage().

Issue-Id: SCM-1861
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: I664acc8a6a1eb17619fa2fd9b372ea661e537923
Signed-off-by: Jon Mason <jon.mason@arm.com>