s390/kvm: Add check for priviledged SCLP handler

The SCLP instruction is priviledged, so we should make sure that
we generate an exception when it is called from the problem state.

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