o   In image_set_size(), don't truncate the swap file. It isn't holding
    the entire image data anymore.
o   Prepare for changes to image_write(): partitioning schemes call
    image_write() to fill in the sectors they reserved from themselves.
    Since we're talking about a handful of sectors, there's no need to
    have them in the swap file. Change struct chunk to accomodate for
    that. A chunk now has a type. That type can be "zeroes", "file" or
    "memory".
o   Flesh out the cleanup function: iterate over the chunks and close
    or free the corresponding data container. Free the chunk as well.