s390/kvm: fix psw rewinding in handle_skey

The PSW can wrap if the guest has been running in the 24 bit or 31 bit
addressing mode. Use __rewind_psw to find the correct address.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>