vdpa/snet: support the suspend vDPA callback

When suspend is called, the driver sends a suspend command to the DPU
through the control mechanism.

Signed-off-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
Message-Id: <20230413073337.31367-3-alvaro.karsz@solid-run.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>