simplify delete logic

Always go to StopVM, which will check the VM status and choose the
appropriate next step.

Change-Id: Ib0fe25fbd82467ac75b6ce478210e5929fe0c46c
hide missing routers from users of VmManager

Let the VmManager deal with a missing router by adding a GONE state. In
the state machine, when we see GONE during an Alive check, delete the
router.

Change-Id: I422620ce1ed76fec3b911720482be23df56a8290
Update state diagram

Change-Id: I5209d55568d10c9c2237e488412a2cb13a4406a6