libxl: unconst the event argument to the event_occurs hook.

libxl: unconst the event argument to the event_occurs hook.

The event is supposed to become owned, and therefore freed, by the application
and the const prevents this.

Unfortunately there is no way to remove the const without breaking existing
callers. The best we can do is use the LIBXL_API_VERSION provisions to remove
the const for callers who wish only to support the 4.3 API and newer.

Callers who wish to support 4.2 will need to live with casting away the const.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>