virtio/s390: use dev_to_virtio

Use dev_to_virtio() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Message-Id: <912bf59bd3a48f2d4d4994681e898dc084fe29d3.1451484163.git.geliangtang@163.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>