VMs don't display serial console on framebuffer without also using the CLI

For both alpine and windows VMs, I've observed that the serial console will not display on the framebuffer without also connecting to the serial console over the propolis CLI with the `serial` command.

I haven't dug into this behavior too much, but it's worth understanding what the interaction is there, because as an end user, I wouldn't expect I need to connect with the CLI to use VNC. 