I've missed some of the discussion, but wouldn't tar be an obvious
choice? It
can expand easily out to a directory hierarchy containing config
file and
multiple virtual disk files, there are standard tools that can
manipulate it
and standard libraries that can be used by Qemu in order to get at the
contents. Only problem I see with this approach is that sparse
file handling
might get a bit strange (using real sparse files vs using tar's
represesntation of sparse files vs compatibility with tars that
don't support
them!).