KVM: s390: fix diagnose code extraction

The diagnose code to be used is the contents of the base register (if not
zero), plus the displacement. The current code ignores the base register
contents. So let's fix that...

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>