virtio/s390: use vring_create_virtqueue

The commit 2a2d1382 ("virtio: Add improved queue allocation API")
establishes a new way of allocating virtqueues (as a part of the effort
that taught DMA to virtio rings).

In the future we will want virtio-ccw to use the DMA API as well.

Let us switch from the legacy method of allocating virtqueues to
vring_create_virtqueue() as the first step into that direction.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>