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 <mreitz@redhat.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1449136891-26850-4-git-send-email-tubo@linux.vnet.ibm.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
