s390x/kvm: unknown DIAGNOSE code should give a specification exception

As described in CP programming services an unimplemented DIAGNOSE
function should return a specification exception. Today we give the
guest an operation exception.
As both exception types are suppressing and Linux as a guest does not
care about the type of program check in its exception table handler
as long as both types have the same kind of error handling (nullifying,
terminating, suppressing etc.) this was unnoticed.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
