KVM: s390: sigp: split handling of SIGP STOP (AND STORE STATUS)

In preparation for further code changes (e.g. getting rid of action_flags),
this patch splits the handling of the two sigp orders SIGP STOP and SIGP STOP
AND STORE STATUS by introducing a separate handler function for SIGP STOP AND
STORE STATUS.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>