[Libosinfo] OS variants

[Zeeshan Ali (Khattak)]

Here is a patch series that adds support for OS variant info (e.g RHEL client & server variants and Windows editions). As a starter, I've added variants to win7 entry and mapped its medias to appropriate editions. I also patched osinfo-detect so if you run it on your win7 ISO (that was already recognised), it should tell you the exact edition of win7 of the media.

There were some medias for which I didn't have the variant info handy so I've left them out for now.

Oh and these patches go on top of 'release status' patch series sent previous to this. I can rebase accordingly..