Add wlr_output.serial
Test plan: you should see something similar in your logs:
2017-11-11 19:04:28 - [rootston/output.c:212] Output 'VGA-1' added
2017-11-11 19:04:28 - [rootston/output.c:215] Samsung Electric Company SyncMaster HS3P701105 474mm x 296mm
Updates #403 (closed)