(Reuters) - International Business Machines Corp on Tuesday reported a surprise rise in revenue, its first in six quarters, benefiting from its high-margin cloud computing business.Revenue from the cloud business, which is crucial as IBM pivots away from its established businesses including mainframe servers, rose 21% to $6.8 billion (5.2 billion pounds) in the fourth quarter ended Dec. 31.Over the past few years, Chief Executive Officer Ginni Rometty has been trying to shift the company's..