s390x/virtio-hcall: Specification exception for illegal subcodes

So far, the DIAG 500 hypervisor call was only setting -EINVAL in
R2 when a guest tried to call this function with an illegal subcode.
This patch now changes the behavior so that a specification exception
is thrown instead, since this is the common behavior of other DIAG
functions (and other CPU instructions) when being called with illegal
parameters.

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