The plane is called the "Phoenix", the "Void Dragon" bit is use to denote that A: It is used by the void dragon corsairs and B: As it is used by the void dragon corsairs it has a weapon loadout unique to the void dragon corsairs (star cannons instead of a pulse laser).
The void dragons are a group of eldar pirates which lend their support out to both craftworld and dark eldar raiding parties for spoils/eldar monies (does that exist?)/personal reasons/debts/ect. The Void Dragon Pheonix came out before there were ally rules and making it available to both eldar and dark eldar was forgeworld's way of having allies without having allies. Rather mercenaries, but ones who only work for the eldar.
Also not to be confused with The Void Dragon, aka the dragon of mars, a surviving sleeping/dormant C'tan.
And I totally didn't answer your question. As for its availability to the dark eldar I would say that since it's a forgeworld piece it is already limited to friendly games so why not? It doesn't look particularly like breaking piece or anything. (is it? I don't actualy know the rules...)