qapi/events: add #if conditions to events

Wrap generated code with #if/#endif using an 'ifcontext' on
QAPIGenCSnippet objects.

This makes a conditional event's qapi_event_send_FOO() compile-time
conditional, but its enum QAPIEvent member remains unconditional for
now. A follow up patch "qapi-event: add 'if' condition to implicit
event enum" will improve this.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180703155648.11933-11-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>