As far as the internet exceeds, Xerneas and Yveltal (and Zyrgade, if you wish) are shiny locked. No one on the internet has been going all psycho about their new shiny legendary. But this has been quoted:
As far as I know, Xerneas and Yveltal are, in fact, locked.
And even if it has been confirmed or not, no major glitches have been unlocked from preventing the Shiny Zekrom and Reshiram from being obtainable, so there is a good chance it probably is locked.
Hope I Helped. :)