The private equity firm that controls Southborough-based IT outsourcer Virtusa Corp. has picked a new chief executive to run the firm and replace one of its founders. Virtusa announced that Santosh Thomas is taking over for cofounder Kris Canekeratne, who is leaving the company. Thomas most recently served as president of global growth markets at Cognizant, a tech outsourcer based in New Jersey. His LinkedIn profile shows that he worked for Cognizant for more than 20 years before leaving in September, and he is based in New York. Virtusa had been publicly traded until February, when private equity firm Baring Private Equity Asia completed its acquisition of the company. It employs more than 22,000 people around the world, and it reported $1.3 billion in revenue for its last full year as a public company.