Add event callback implementation based on virConnectDomainEventRegisterAny

Add event callback implementation based on virConnectDomainEventRegisterAny

A client may register a callback as follows:

    E.register_default_impl ();

    let conn = C.connect_readonly ?name () in

    let id = E.register_any conn (E.Lifecycle (fun dom e ->
        printd dom "Lifecycle %s" (E.Lifecycle.to_string e)
    )) in

Internally this will:
  1. generate a unique int64 used to identify the specific callback
  2. add the callback to an OCaml hashtable based on the signature
     (there is a distinct hashtable per callback signature)
  3. call virConnectDomainEventRegisterAny which registers a
     generic C callback in the stubs (one distinct callback per
     signature) and supply the int64 as the "opaque" data

The client must enter the event loop with:

    while true do
        E.run_default_impl ()
    done

When an event is triggered, the C callback will upcall into an OCaml
function (having re-acquired the heap lock) supplying the int64 value.
The OCaml function can then find the right callback in the Hashtbl
and call it.

The client can deregister the callback with:

  E.deregister_any conn id;

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