testdisplay uses GIO for polling the udev fd for events. When shutting
down, both the GIO channel and libudev want to close their respective
fds, leading to a double close.

Since version 240 (as used by Ubuntu 19.04), libudev has complained
loudly if close() fails with EBADF. Previously the error has flown
under the radar.

Calling g_io_channel_shutdown() will call close() on the fd regardless
of g_io_channel_set_close_on_unref, so opencode the flush and unref
instead to avoid double closing.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Jani Saarinen <jani.saarinen@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Tested-by: Jani Saarinen <jani.saarinen@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>