CommonDevice: Fix relative MRL generation
CommonDevice: Fix relative MRL generation

matchesMountpoint will return the mountpoint that matches the currently
analyzed MRL, so we can just remove the entire length of the mountpoint,
rather than performing another comparison, especially since it was done
in a case sensitive way, causing false positive and therefor
concatenating the mountpoint and the absolute mrl.
Fix #341

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary