Read image sizes from imagelist files for non-Pungi 4 composes
    
    When we're synthesizing image dicts for composes where we don't
    have full metadata, we will shortly be able to get the image
    size, as https://pagure.io/quick-fedora-mirror/pull-request/82
    will enhance the imagelist files we use to include the size of
    each image. This handles both the old format and the planned
    new format, reading in the sizes if we have them.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>