Instance specs and machine init disagree about the necessity of COM1

It's technically possible to create an instance spec with no serial device associated with COM1, but if you try, `propolis_server::initializer::MachineInitializer::initialize_uart` will panic. It would be nice to resolve the tension between these modules.

However, we have no scenario where we actually *want* to omit this device for now, so this isn't needed anytime soon.