Add exit notifiers.

Add exit notifiers.

Hook up any cleanup work which needs to be done here.  Advantages over
using atexit(3):

  (1) You get passed in a pointer to the notifier.  If you embed that
      into your state struct you can use container_of() to get get your
      state info.
  (2) You can unregister, say when un-plugging a device.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit fd42deeb4cb42f90084046e3ebdb4383953195e3)
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>