Some commands such as quit or cont have one letter alternatives but
stop is missing that. Add stop|s to match cont|c for consistency and
convenience.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20211030095225.513D4748F48@zero.eik.bme.hu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>