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

[Richard W.M. Jones]

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.