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.

[ v2: move code out of #ifndef _WIN32 ]

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>