[Libguestfs] [PATCH EXPERIMENTAL nbdkit 0/2] Port to Windows using mingw.

The patches following do indeed allow you to compile nbdkit.exe, but it does not actually work yet. I'm posting this experimental series more as a work in progress and to get feedback. Note this does not require Windows itself to build or test. You can cross-compile it using mingw64-* packages on Fedora or Debian, and test it [spoiler alert: it fails] using Wine. Rich.