s390x/kvm: Fixed bad SIGP SET-ARCHITECTURE handler

The SET-ARCHITECTURE handler in QEMU caused a program interruption.
This is wrong according to the "Principles of Operations" specification
(since SIGP should never cause a program interrupt) and was likely only
introduced for debugging purposes. Since we handle SET-ARCHITECTURE in
the kernel already and only dropped to user space in case of bad mode
parameters, we should just report INVALID PARAMETER in QEMU instead.

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>
