vdpa/snet: implement kick_vq_with_data callback

Implement the kick_vq_with_data vDPA callback.
On kick, we pass the next available data to the DPU by writing it in
the kick offset.

Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
Message-Id: <20230417083853.375076-1-alvaro.karsz@solid-run.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>