libcec: Make verbose logging optional

On a Pi without a CEC capable TV, you get about 4 log messages a second:
```
17:41:25 11830.849609 T:2930766928   DEBUG: CecLogMessage - >> POLL not sent
17:41:26 11831.850586 T:2930766928   DEBUG: CecLogMessage - << Recorder 1 (1) -> TV (0): POLL
17:41:26 11831.851562 T:2930766928   DEBUG: CecLogMessage - << 10
17:41:26 11831.943359 T:2930766928   DEBUG: CecLogMessage - command 'POLL' was not acked by the controller
```
Disable the verbose messages by default, and allow them to be optionally enabled.