HONG KONG, July 21 (Reuters) - China's two main bourses on Friday issued guidelines for rating the performances of sponsor institutions such as brokerages who help firms list on the exchanges, as part of efforts to battle capital market fraud. Sponsors would be assessed by the "quality" of listed firms they assisted, such as management competency, market performance and whether firms were operating under sound internal governance, according to a statement jointly issued by the Shanghai stock exchange and Shenzhen stock exchange. The other criteria in evaluating sponsors is how they perform in assisting firms in the initial public offering...