<p>Virtu Financial Inc said it would buy rival KCG Holdings Inc in a $1.4 billion deal that brings together two major U.S. electronic trading and market-making firms. Virtu Financial Inc said it would buy rival KCG Holdings Inc in a US$1.4 billion deal that brings together two major U.S. electronic trading and market-making firms.The deal comes at a difficult time for high-frequency trading as low volatility squeezes the profits it can make from rapid-fire trades.</p>