libxl: New event generation API

libxl: New event generation API

Replace the existing API for retrieving high-level events (events
about domains, etc.) from libxl with a new one.

This changes the definition and semantics of the `libxl_event'
structure, and replaces the calls for obtaining information about
domain death and disk eject events.

This is an incompatible change, sorry.  The alternative was to try to
provide both the previous horrid API and the new one, and would also
involve never using the name `libxl_event' for the new interface.

The new "libxl_event" structure is blacklisted in the ocaml bindings
for two reasons:
  - It has a field name "type" (which is a keyword in ocaml);
    the ocaml idl generator should massage this field name on
    output, to "type_" perhaps.
  - The ocaml idl generator does not support KeyedUnion.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>