Let's support reading and writing of control registers for kvm and tcg.

We have to take care of flushing the tlb (tcg) and pushing the changed
registers into kvm.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>