Melt margarine in a medium saucepan over medium heat. Add onion, and saute until tender. Sprinkle flour over onion, and cook over low heat, stirring constantly, 1 minute. Gradually add chicken broth and wine; cook over medium heat, stirring constantly, until mixture is thickened and bubbly.
Stir in sweet potato, ginger, and salt. Reduce heat, and simmer, uncovered, 30 minutes or until potato is tender. Stir in milk and lime juice. Transfer sweet potato mixture to container of an electric blender; cover and process until smooth.
To serve, ladle soup into individual bowls, and top each serving with 1 tablespoon cranberries.