Fix glib's 'source id not found' warning

When glib.io_add_watch's callback function returns False, the source_id is
automatically removed from his list. By not returning False on the _io_watch
function, glib will be forced to remove his source_id when it commanded to
do so (on _finished function).

Change-Id: Ic6cb8221159e96f9377da720b53352dac7ab33c9