Disable VM serial port access

[aumohr]

Describe the bug
The organization policy "Disable VM serial port access" is deactivated by default even though the enforcement is recommended by GCP security best practices.

Rationale
GCP implements the serial port as an Internet accessible SSH service running on port 9600 on ssh-serialport.googleapis.com without any ability to set firewall rules; you just require an SSH metadata key that is set for that instance or project.

Google recommends disabling serial ports as it presents the same security issue as an SSH server open to 0.0.0.0/0.

Recommendation
Disable VM serial port access. [by organization policy]