He almost certainly has not been cast, and is highly unlikely to be. He is often mentioned and enough bits have been given about his background (basically, he's doing in Westeros what Viserys is doing across the sea with Illyrio and the Dothraki - trying to obtain military support to take back his crown, but no one really gives a damn - they keep him around for the exotic appeal and to show off at parties, and otherwise ignore) to flesh him out, but he plays no real role in the story and may not actually have any lines of dialogue in the series. At most people mention that he greeted someone or that he asked the crown for aid or something. The ship and crew that take Sam, Maester Aemon and the wildling girl from Braavos to Oldtown in aFfC is from Xho's homeland, and their sole appearance in half of a single character's story arc from a single book is many times the amount of attention Xho receives.