Among the players born in Whitestone, how many of them have won the MVP?
"Whitestone" is the birthCity of the player; won the MVP refers to award = 'Most Valuable Player'
SELECT COUNT(DISTINCT T1.playerID) FROM players AS T1 INNER JOIN awards_players AS T2 ON T1.playerID = T2.playerID WHERE T2.award = 'Most Valuable Player' AND T1.birthCity = 'Houston'