libxl: ocaml: use 'for_app_registration' in osevent callbacks

This allows the application to pass a token to libxl in the fd/timeout
registration callbacks, which it receives back in modification or
deregistration callbacks.

It turns out that this is essential for timeout handling, in order to
identify which timeout to change on a modify event.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>