Try this code:

 # here I get the coordinates of the button relative to
 # window (self.window)
 button_x, button_y = self.actions_button.allocation.x, self.actions_button.allocation.y

 # now convert them to X11-relative
 window_x, window_y = self.window.window.get_origin()
 x = window_x + button_x
 y = window_y + button_y

now x, y hold the top-left corner of button and those x, y are X11 and not window-relative.

Something more advanced but hopefully useful:

 menu.popup(None, None, self.position_actions_menu, 1, 0)
 menu.show_all()

 def position_actions_menu(self, menu):
	# here I get the coordinates of the button relative to
	# window (self.window)
	button_x, button_y = self.actions_button.allocation.x, self.actions_button.allocation.y

	# now convert them to X11-relative
	window_x, window_y = self.window.window.get_origin()
	x = window_x + button_x
	y = window_y + button_y

	# now move the menu below the button
	y += self.actions_button.allocation.height

	push_in = True # push_in is True so all menu is always inside screen
	return (x, y, push_in)