KVM: s390: Fix sdnxo setting for nested guests

If the guest does not use the host register management, but it uses
the sdnx area, we must fill in a proper sdnxo value (address of sdnx
and the sdnxc).

Reported-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>