avoid StoragePool.getvirtobj() in StorageVolume

Method StorageVolume().find_storage_volume() was improperly using
StorageVolume.getvirtobj() which sometimes returns None. All these
getvirtobj() methods should be avoided as much as possible now that
there are these new reetrant find_*() replacements.
add new method Cloubed.cleanup()

This also includes {Domain,Network,StorageVolume,StoragePool}.destroy()
new cleanup action for cloubed script