LONDON (Reuters) - At least two top 10 shareholders in miner Xstrata plan to vote against a takeover by commodities trader Glencore, threatening the creation of a powerhouse spanning mining, agriculture and trading. Standard Life Investments and Schroders said on Tuesday the deal, the mining sector's biggest, to buy the remaining 66 percent of Xstrata for $41 billion, undervalued their shares. The deal, designed to create a company to rival mining heavyweights such as BHP Billiton and Rio Tinto, needs to be approved by 75 percent of shareholders excluding Glencore, which is barred from voting. ...