s390x: use migrate_once() in migration tests

migrate_once() can simplify the control flow in migration-skey and
migration-cmm.

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20221212111731.292942-4-nrb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>