(Recasts with Iran's request for U.N. action, details throughout) By Michelle Nichols UNITED NATIONS, April 22 (Reuters) - Iran asked the United Nations on Tuesday to demand that the United States grant a visa to Tehran's proposed new U.N. envoy, while Washington stood firm on its decision to deny entry to Hamid Abutalebi over his suspected links to hostage-takers. The United States has said it would **t grant a visa to Abutalebi because of his connection to the 1979-1981 Tehran hostage crisis when Iranian students seized the U.S. Embassy and held 52 Americans hostage for 444 days.