How to handle this kind of non-linear constraint in the optimization problem?

Question

I have an optimization problem where I have following constraint which is causing problem in converting the problem into a convex optimization problem. The constraint is given as follows $$g\geq \frac{1}{a+\|\textbf{x}\|^2}$$ where $\textbf{x}$ is a two dimensional column vector and $a>0$ is a constant. In this constraint the optimization variables are $g$ and $\textbf{x}$. I do not know how to convexify this constraint. I think if I can find a convex upper bound of $$\frac{1}{a+\|\textbf{x}\|^2}$$ then the above constraint can be converted into a convex set. However, I do not know how to get the convex upper bound of $\frac{1}{a+\|\textbf{x}\|^2}$. Any help in this regard will be much appreciated. Thanks in advance.