* Slightly positive on capital (Adds details, background, comment) : MADRID (Reuters) - Spain’s BBVA (BBVA.MC) said on Wednesday it had agreed to sell 80 percent of its real estate business to U.S. fund Cerberus for 4 billion euros ($5 billion), showing how investor enthusiasm for Spanish property is reviving. A burst property bubble in 2008 sent Spain into a downturn that lasted nearly five years, causing mass unemployment and prompting a more-than 40-billion-euro bailout for the country’s banks.