qemu-iotests: s390x: fix test 068

qemu-iotests: s390x: fix test 068

Now, s390-virtio-ccw is default machine and s390-ccw.img is default boot
loader. If the s390-virtio-ccw machine finds no device to load from and
errors out, then emits a panic and exits the vm. This breaks test cases
068 for s390x.
Adding the parameter of "-no-shutdown" for s390-ccw-virtio will pause VM
before shutdown.

Acked-by: Max Reitz <[email protected]>
Reviewed-by: Sascha Silbe <[email protected]>
Signed-off-by: Bo Tu <[email protected]>
Message-id: 1449136891[email protected]
Signed-off-by: Max Reitz <[email protected]>