[Libguestfs] [PATCH libnbd v4] Add Go language bindings (golang) (RHBZ#1814538).

Now runs a complete set of tests, notably including the AIO test. File descriptors are passed in and out as plain ints (instead of *os.File) for a couple of reasons: (1) We have to pass the plain int to syscall.Select. (2) Turning an fd into an os.File causes golang to set the blocking flag which is deeply unhelpful. Rich.