db: Take the media's volume size into account when sorting the medias
If volume-size is not taken into account, when dealing with identical ISOs, we may end up wrongly matching an entry that has its volume-size with one which doesn't have the value, simply because the one which does not have the value may be declared first the in XML entry.
Signed-off-by: Fabiano Fidêncio fidencio@redhat.com