Serial console should use websockets

[just-be-dev]

Once the buffering capabilities are added to the websocket console stream we should swap the implementation to use the websockets endpoint instead of the current GET endpoint. This should be a passthrough in xterm which will reduce the overall complexity of maintaining console state.

[david-crespo]

I asked @lifning about the status of the API work. The websocket endpoint is already there in Nexus, but the ring buffer in Propolis containing the last N bytes of history is not in yet (it's in propolis but omicron can't be upgraded yet). This means you can watch the serial console live with the websocket endpoint, but if you disconnect and reconnect, you will only see stuff from the present on.

The non-streaming endpoint gives you the history, so for now it's useful for debugging. We will wait to change the web console over to the streams endpoint until the ring buffer is in.