Disconnect event on vivid is not handled properly

With a `vivid` device proxied on the host:

```
$ insmod virtio-media.ko driver_name=vivid
$ v4l2-compliance -d0
...
                fail: v4l2-test-controls.cpp(1226): doioctl(node, VIDIOC_S_CTRL, &ctrl)
        test Disconnect: FAIL
```

We need to define what a 'disconnect' means for us and how to signal it to the guest - probably an error event?