[Libguestfs] [PATCH libnbd] copy: Refactor struct rw

This is a large refactoring change which tries to clean up the internals of nbdcopy by making the various "ops" truly encapsulated. It obviously will create massive conflicts with Nir's patch series (although almost completely mechanical to fix). I promise to resolve those conflicts for you if you get me an updated version of your patch series. Rich.