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

[Richard W.M. Jones]

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.