iotests: use -ccw on s390x for 051

iotests: use -ccw on s390x for 051

The default cpu model on s390x does not provide zPCI, which is
not yet wired up on tcg. Moreover, virtio-ccw is the standard
on s390x, so use the -ccw instead of the -pci versions of virtio
devices on s390x.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: QingFeng Hao <haoqf@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>