s390x/ioinst: set type and len for SEI response

If no event information is pending, the return code
is set to 0x0005 and the length of the response is
set to 8 bytes.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Song Shan Gong <gongss@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>