Link to ocamlfind package 'xen-event' for the Eventchn module

The same OCaml code can be used for both userspace and kernelspace.

We keep the C stubs in-tree for now.

We also remove the hack where we advertised a 'xenctrl' package.
Hopefully out-of-tree drivers can depend only on xen-event.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>