hw/char: Have FEWatchFunc handlers return G_SOURCE_CONTINUE/REMOVE

hw/char: Have FEWatchFunc handlers return G_SOURCE_CONTINUE/REMOVE

GLib recommend to use G_SOURCE_REMOVE / G_SOURCE_CONTINUE
for GSourceFunc callbacks. Our FEWatchFunc is a GSourceFunc
returning such value. Use such definitions which are
"more memorable" [*].

[*] https://docs.gtk.org/glib/callback.SourceFunc.html#return-value

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230705133139.54419-5-philmd@linaro.org>