By Parisa Hafezi VIENNA (Reuters) – Talks to revive Iran’s 2015 nuclear deal with world powers were mired in uncertainty on Sunday following Russia’s demands for a U.S. guarantee that the sanctions it faces over the Ukraine conflict will not hurt its trade with Tehran. Moscow threw the potential spanner in the works on Saturday, just as months of indirect talks between Tehran and Washington in Vienna appeared to be headed for an agreement, with Foreign Minister Sergei Lavrov saying the Western sanctions over Ukraine had become a stumbling block for the nuclear deal. U.S. Secretary of State Ant…