Valdes GK def
pique, Mascherano CB Def
Alba, FB def
Alves, WB Attack, wide play hug touchline
Busquets, Anchor man def
Xavi, DLP support
Iniesta, CM support
Messi Trequartista?!?
Pedro, Winger Attack, Wide play hug touchline
Villa, poacher/advanced forward.

This would be my guess based on those positions however it would all depend on how the team played outside of one still image...
i go to his thread, the tactic isn't look like in a picture that i posted
the tactic doesnt no, however if you read the information in the thread it will help you build yourself a tactic using the idea you posted above.
The thread is absolutely fantastic for anyone who wishes to pursue a possession based approach, which in essence is how Barca play
okay, i have question.. how to make wingers track back ?? and if alba's job "stayed in position and proved very useful as a defender" what role and duty for him ??
i quote that from the article
to make your wingers track back I think you would need to set up man marking instructions to the opposition full back/wing back or winger if it is this player you want covered.

To get Alba to stay back you would set him as full back defend. This will set his creative freedom to low, have roam from position unticked and his closing down to own area.
then, about xavi and iniesta, are their roles are APs for xavi and APa for iniesta ???
I think from the screenie you posted, Xavi is a Deep Lying Playmaker
I'm unsure about Iniesta, probably he is an AP.
okay i'll try with that,
wait, i read in the article, villa's duty in the match is "trying to find space between the centre-backs". is that duty for Trequartista ??
No mate. I saw that game and Villa's job was maintain both centre-backs occupied. So I'll go for Advanced Forward or Poacher. As trequartista he would drop deeper and that's not his purpose. DLF was also a good option but I guess it is not what you want for your recreation..
No the Trequartista moves around alot, creating space and dragging players,

"space between the centre-backs" would be a Poacher. but I would change the player instruction a little. tick runs with ball and change it to rarely.
if poacher/adv. forward role for villa, whats the right instructions for him to attack and support messi??
Set Messi as Trequartista AMCR, run from deep often, run with ball often, Playmaker, Target Man (supply to feet) and let him get on with it.

Everything else is kind of irrelevant.

Funny thing is, I'm not even joking. ;)