Function attach_device 

pub async fn attach_device<P, S>(
    path: P,
    socket: S,
    block_size: u32,
    block_count: u64,
    read_only: bool,
) -> Result<Server>
where
    P: AsRef<Path>,
    S: AsRawFd + Send + 'static,

Attach a socket to a NBD device file.