s390/cpcmd: use physical address for command and response

Virtual Console Function DIAGNOSE 8 accepts physical
addresses of command and response strings.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>