So, here's the answer:

Along with all the instruments wave data, there are recorded hundreds of guitar riffs and chords in all tonalities, but... only Maj and Min chords are present, with some exceptions.
So, for the most part of guitar loops you got only Maj and Min versions. How do they create other chords? Simply... for chords that have note added (lik 6, 7, 9...) they simply add a MIDI note in the correct position along with the chord. For example if you play a C7, you get the CMaj guitar along with a midi guitar that plays the A# note. If you play CMin7, the same... you get the Min chord + the MIDI A# note.
But for other chords that don't have additions, like C4, Cdim, CAug for example, all the loop is played by MIDI and you can clearly hear the live guitar absence in some styles where the live guitar is predominant.
as I told before, I found a few exceptions (bossa and samba riffs) where playing a 7th chord brings up a different sample instead of adding the midi note.
Other samples, notably the rock ones have only one version played regardless of Maj or Min chords.

[This message has been edited by mrdave (edited 02-01-2009).]