sqrtm is not supported by CVX. If this is a convex optimization problem, you will need some other formulation.
In the future, before asking how to use a function in CVX, pleas look to see if it is listed in the CVX Users’ Guide.
Extension of MATLAB functions for CVX
http://cvxr.com/cvx/doc/funcref.html#built-in-functions
New functions for CVX
http://cvxr.com/cvx/doc/funcref.html#new-functions