qapi: Generate QAPIEvent stuff into separate files

qapi: Generate QAPIEvent stuff into separate files

Having to include qapi-events.h just for QAPIEvent is suboptimal, but
quite tolerable now.  It'll become problematic when we have events
conditional on the target, because then qapi-events.h won't be usable
from target-independent code anymore.  Avoid that by generating it
into separate files.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190214152251.2073-6-armbru@redhat.com>