[Libguestfs] New Rust bindings for nbdkit

The existing Rust bindings for nbdkit aren't very idiomatic Rust, and they are missing a lot of features. So I've rewritten them. The new bindings aren't backwards compatible, but I doubt that's a problem. Most likely, nobody has tried to use them yet, since the crate hasn't even published to crates.io. Please review the attached patch. -Alan