iotests: Fix test 200 on s390x without virtio-pci

iotests: Fix test 200 on s390x without virtio-pci

virtio-pci is optional on s390x, e.g. in downstream RHEL builds, it
is disabled. On s390x, virtio-ccw should be used instead. Other tests
like 051 or 240 already use virtio-scsi-ccw instead of virtio-scsi-pci
on s390x, so let's do the same here and always use virtio-scsi-ccw on
s390x.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>