The trading arms of some of China’s biggest state-owned oil giants have started selling crude for May loadings in a rare move from the majors that have cut refinery rates in response to soaring oil prices and constrained crude supply from the Middle East. Sinopec, the world’s biggest refiner by capacity, and Sinochem Group have already sold crude grades from Nigeria and Ghana, mostly to refiners in Asia, including Indonesia and Taiwan, anonymous traders with knowledge of the deals told Bloomberg on Wednesday. The state refiners in China…