Creating a vhost-user-vsock backend

Creating a vhost-user-vsock backend

[DUO Labs]

I've been studying the example given in `contrib/vhost-user-blk`, and trying to match the code to what is in the standard (https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-3050002), so I can use the code to create my own vhost-user-vsock server.

I think I almost understand it, but I don't get exactly how do you correlate out_sg and in_sg to the header and ancillary data (for virtio-blk, it would either be the data to be written or a buffer to read data into, and vsock would be similar), respectively.

In virtio-blk, I see that there's an inhdr, and outhdr, but vsock doesn't have that, so I'm not sure how to map the same concept. Similarly, does `elem->out_sg[0]` always map to the header, and `elem->out_sg[1]` always map to the ancillary data? What about when you are reading into a buffer --- should it always be read into `elem->in_sg[0]`, or is that only for virtio-blk?