[Virtio-fs] [virtiofsd-rs] MR opened: Add vhost-user JSON file

[virtiofs-bot at sinrega.org]

Add a JSON file as defined by QEMU's docs/interop/vhost-user.rst in
the "Backend program conventions" section.

This is expected to be installed in
$DATADIR/qemu/vhost-user/50-qemu-virtiofsd.json

Signed-off-by: Sergio Lopez <slp at redhat.com>
---
https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/47