Why isn't cvx accepting $log(x^t*A*x)$, where A is positive definite?

[Mark_L_Stone]

Please read the FAQ Why isn't CVX accepting my model? READ THIS FIRST!.

help cvx/log

Disciplined convex programming information:
    log(X) is concave and nondecreasing in X. When used in CVX
    expressions, X must be concave.