Serial port option prohibits saving settings even when serial port disabled

[bleh]

If a Serial Port is disabled, it is expected that even if it contains invalid settings, it will not prohibit saving guest settings.

To reproduce:

1. Open settings for any guest
2. Select "Serial Ports" in left pane
3. Check "Enable Serial Port"
4. For Port Mode, select "Host Pipe" and note that status reads "Invalid settings detected"
5. Uncheck "Enable Serial Port"
6. Note that the OK button is grayed out.