Sharing QEMU devices via vhost-user protocol.

Only one vhost-user client can connect to the server one time.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Coiby Xu <coiby.xu@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20200918080912.321299-4-coiby.xu@gmail.com
[Fixed size_t %lu -> %zu format string compiler error.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>