virb-track.gpx is the original gpx file that you got when you imported from a VIRB.
virb.gpx is the current track for the clip. (Could be the same one, or it could be one you manually added or replaced.)
In many cases, they are the exact same file. But we wanted to have a backup in case the user replaced the original track on accident. Having virb-track.gpx around will allow the user to retrieve the old track. (There is no UI for retrieving the track in 2.2 yet, but there will be in 2.3).