qapi-event: Utilize implicit struct visits

qapi-event: Utilize implicit struct visits

Rather than generate inline per-member visits, take advantage
of the 'visit_type_FOO_members()' function for emitting events.
This is possible now that implicit structs can be visited like
any other.  Generated code shrinks accordingly; by initializing
a struct based on parameters, through a new gen_param_var()
helper, like:

|@@ -338,6 +250,9 @@ void qapi_event_send_block_job_error(con
|     QMPEventFuncEmit emit = qmp_event_get_func_emit();
|     QmpOutputVisitor *qov;
|     Visitor *v;
|+    q_obj_BLOCK_JOB_ERROR_arg param = {
|+        (char *)device, operation, action
|+    };
|
|     if (!emit) {
|         return;
@@ -351,19 +266,7 @@ void qapi_event_send_block_job_error(con
|     if (err) {
|         goto out;
|     }
|-    visit_type_str(v, "device", (char **)&device, &err);
|-    if (err) {
|-        goto out_obj;
|-    }
|-    visit_type_IoOperationType(v, "operation", &operation, &err);
|-    if (err) {
|-        goto out_obj;
|-    }
|-    visit_type_BlockErrorAction(v, "action", &action, &err);
|-    if (err) {
|-        goto out_obj;
|-    }
|-out_obj:
|+    visit_type_q_obj_BLOCK_JOB_ERROR_arg_members(v, &param, &err);
|     visit_end_struct(v, err ? NULL : &err);

Notice that the initialization of 'param' has to cast away const
(just as the old gen_visit_members() had to do): we can't change
the signature of the user function (which uses 'const char *'), but
have to assign it to a non-const QAPI object (which requires
'char *').

While touching this, document with a FIXME comment that there is
still a potential collision between QMP members and our choice of
local variable names within qapi_event_send_FOO().

This patch also paves the way for some followup simplifications
in the generator, in subsequent patches.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1458254921-17042-8-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>