[Libguestfs] [PATCH nbdkit] include: Annotate function parameters with attribute((nonnull)).

[Richard W.M. Jones]

Should we use attribute((nonnull)) at all?  There's a very interesting
history of this in libvirt -- try looking at commit eefb881 plus the
commits referencing eefb881 -- but it does seem to work for me using
recent GCC and Clang.

I only did a few functions because annotating them gets old quickly...

Rich.