KVM: s390: add documentation for diag 501

Added documentation for diag 501, stating that no subfunctions are provided and
no parameters are used.

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