[Libguestfs] Rust Bindings for Libnbd

This is Tage. I am a Google Summer of Code contributer this summer working on Rust bindings for Libnbd. So far, I have made a very basic first draft which is far from complete. But I'll send the patches anyway so you can take a look. I have created "generator/Rust.ml" which generates Rust code for constants, flags, enums and handle calls. The top level "rust/" directory contains the actual Rust crate. The make script generates documentation with Rustdoc which can be found in "rust/target/doc/libnbd/index.html". To generate low level bindings, [rust-bindgen](https://github.com/rust-lang/rust-bindgen) is used. This means that Clang is required to build the bindings. I plan on removing this dependency in the future though. Feel free to send any comments and ideas, butI emphasize again that this is just a first draft, and there is much more work to do. Best regards, Tage