KVM: s390: fix stsi exception handling

In case of an exception the guest psw condition code should be left alone.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-By: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>