Is novelty requirement for PhD restricts the selection of research question?
Well, it is slightly unlikely that a novel researcher will solve one of the very fundamental problems in their PhD. 

But whilst working on a solution, you will most probably find different other intersting problems which are unsolved or novel an those can be published and can be the basis of a PhD thesis. 

You will have to convince a supervisor that you 


understood the problem well
have some novel ideas which have not been tried before


and with this you may find someone. But be prepared for an intense discussion. It hppens quite often that a student claims to have a solution for a very fundamental problem and after half an hour of discussion the idea is gone with the wind... A good supervisor wants to avoid such a situation during your PhD.