media-manager: Rename a method

Rename compare_media() to compare_media_by_label().

https://bugzilla.gnome.org/show_bug.cgi?id=685243
media-manager: Avoid listing duplicate media

In case of duplicate media, prefer:

* released OS over unreleased one
* latest release
* soft over hard media

https://bugzilla.gnome.org/show_bug.cgi?id=685243