want stop requests to inform the guest OS so that it can try to shut down gracefully

[gjcolombo]

Today, asking a running Propolis to stop does not assert any signals into the guest (like a virtual power button press) that give the guest OS a chance to run its own shutdown logic. It would be nice if Propolis did assert such a signal, with a timeout (preferably configurable, at least through the TOML/command line, if not by the HTTP API) after which it will proceed down its existing, non-graceful instance-stop path.