virtio-vdpa: don't set callback if virtio doesn't need it

There's no need for setting callbacks for the driver that doesn't care
about that.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-3-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>