[fix] Uninstall services correctly

this is an attempt to fix http://trac.xbmc.org/ticket/13620, works fine here

2 commits:
1) check if another instance of the service is running before starting it
2) stop the service before uninstalling it/deleting it
3) cosmetics, duplicate code
4) correctly handle service addon rollback process

@MartijnKaijser , @ronie if you could test please