mem_event: use C99 initializers for mem_event_request_t users

mem_event: use C99 initializers for mem_event_request_t users

Use C99 initializers for mem_event_request_t users to make sure req is
always cleared, even with local debug patches that shuffle code around
to have a single exit point.
The common case is to use and send req, so it does not add significant
overhead to always clear req.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>