hw/i386/pc: Unify vmport=auto handling

hw/i386/pc: Unify vmport=auto handling

The code which translates vmport=auto to on/off is currently separate
for each PC machine variant, while being functionally equivalent.
This moves the translation into a shared initialization function, while
also tightening the enum assertion.

Signed-off-by: Kamil Szczęk <kamil@szczek.dev>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <v8pz1uwgIYWkidgZK-o8H-qJvnSyl0641XVmNO43Qls307AA3QRPuad_py6xGe0JAxB6yDEe76oZ8tau_n-2Y6sJBCKzCujNbEUUFhd-ahI=@szczek.dev>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>