It appears that you have a typo. rel_entr(x,x+y) <= (positive real constant) is a convex constraint and is accepted by CVX. However, x*log(1+(y/x)) - (positive real constant) = -rel_entr(x,x+y) - (positive real constant). Therefore, apparently you meant -rel_entr(x,x+y) <= (positive real constant), and that is an inherently non-convex constraint (the inequality is going the wrong direction to be convex), for which there is no reformulation to convex constraint.