Um... isn't that guaranteed? What would it mean for a private function to be virtual? It can't be overridden by a different implementation in a child class...
I too thought that sounded insane, so I just looked it up. I've been programming C++ for twenty five years and the thought of wanting to do this have never ever occurred to me...
This is, admittedly, a pretty niche case but certainly not inconceivable.
Making all virtual functions private is the better way to implement inheritance: it separates implementation from interface, allows for common functionality to be moved to the base class without requiring gymnastics in each and every derived class, and even lets you simplify the public interface to reduce compile times.
Try it, you'll like it. You may never go back to writing Java in C++.