Function serve_local_nbd 

pub async fn serve_local_nbd<P, B>(
    path: P,
    block_size: u32,
    block_count: u64,
    read_only: bool,
    block_device: B,
) -> Result<()>
where
    P: AsRef<Path>,
    B: Unpin + BlockDevice,

Serve a local block device through a NBD dev file.