Strain plums through a sieve over a bowl, reserving juice. Cut plums in half; discard pits. Combine plums, reserved juice, red wine, honey, cinnamon, and cloves in a medium saucepan. Bring to a boil. Cover; reduce heat, and simmer 15 minutes. Place half of soup in a blender; process until smooth. Pour into a bowl. Repeat procedure with other half of soup. Stir in liqueur and rind. Place 3/4 cup soup into each of 6 wine goblets. Top each serving with 1 tablespoon yogurt and 1 mint sprig, if desired.