Place onion slices into the bottom of a large crockpot. Dot roast with cloves. Place roast in crockpot along with the water. Cover and cook at least 8 hours (or overnight). Take roast from crockpot, remove cloves and shred pork. Discard liquid and onions from crockpot. Place shredded pork, barbecue sauce and sugar in crockpot and cover. Cook at least 2 more hours. Serve on toasted buns with slices of fresh onion (if desired).