Port virtio media simple device as a vhost user media device.

[ser-io]

Port virtio media reference device simple_device as a vhost user implementation.

First iteration of simple_device/src/simple_device.rs is the same as in virtio-media upstream https://github.com/chromeos/virtio-media/blob/main/device/src/devices/simple_device.rs.

cargo run --bin simple_device  -- --socket-path /tmp/simple_device.sock

cargo run --features "media" -- --log-level=debug run --cpus 4 --mem 4096 \
  --rwdisk /path/to/debian-12.img \
  --params "root=/dev/vda1" \
  --vhost-user media,socket=/tmp/simple_device.sock \
  /path/to/bzImage

v4l2-compliance -d0 -s

Bug: 445229097