Season the pieces of lamb shoulder with salt and black pepper, to taste. In a small bowl mix the chili powder, ground cumin and ground cloves. Season the lamb with the chili powder mix. Put the meat in a slow cooker; add the onion, garlic and coriander. Cover and cook on low for 6-8 hours or until the meat is tender and can be easily pulled apart. Turn off the cooker and shred the meat.