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

scripts/qapi/events.py

@@ -185,8 +185,10 @@ class QAPISchemaGenEventVisitor(QAPISchemaModularCVisitor):
         genc.add(gen_enum_lookup(self._enum_name, self._event_names))
 
     def visit_event(self, name, info, ifcond, arg_type, boxed):
-        self._genh.add(gen_event_send_decl(name, arg_type, boxed))
-        self._genc.add(gen_event_send(name, arg_type, boxed, self._enum_name))
+        with ifcontext(ifcond, self._genh, self._genc):
+            self._genh.add(gen_event_send_decl(name, arg_type, boxed))
+            self._genc.add(gen_event_send(name, arg_type, boxed,
+                                          self._enum_name))
         self._event_names.append(name)