[Libguestfs] [PATCH nbdkit] Add support for writing plugins in Rust.

This adds very rough support for writing nbdkit plugins in Rust. This is not very idiomatic -- essentially we're handling the direct C calls from nbdkit in Rust. We have to use ‘unsafe’ in a few places because there's no way to tell the Rust code that nbdkit satisfies guarantees (eg. around thread safety, always returning leaked pointers back to the close function, always doing bounds checking). See the TODO file changes for the mising bits. TBH there's not much to review here. Rich.