tools lib traceevent: Unregister handler when xen plugin is unloaded

The function handler should be unregistered when the plugin is unloaded
otherwise it'll try to access invalid memory.

Signed-off-by: Namhyung Kim <[e-mail]>
Reviewed-by: Jiri Olsa <[e-mail]>
Acked-by: Steven Rostedt <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/plugin_xen.c

···
 				       PEVENT_FUNC_ARG_VOID);
 	return 0;
 }
+
+void PEVENT_PLUGIN_UNLOADER(struct pevent *pevent)
+{
+	pevent_unregister_print_function(pevent, process_xen_hypercall_name,
+					 "xen_hypercall_name");
+}