qapi: Track all failures between visit_start/stop

Inside the generated code between visit_start_struct() and
visit_end_struct(), we were blindly setting the error into
the caller's errp parameter.  But a future patch to split
visit_end_struct() will require that we take action based
on whether an error has occurred, which requires us to track
all actions through a local err.  Rewrite the visits to be
more in line with the other generated calls.

Generated code changes look like:

|     visit_start_struct(v, (void **)obj, "Abort", name, sizeof(Abort), &err);
|-    if (!err) {
|-        if (*obj) {
|-            visit_type_Abort_fields(v, obj, errp);
|-        }
|-        visit_end_struct(v, &err);
|+    if (err) {
|+        goto out;
|     }
|+    if (!*obj) {
|+        goto out_obj;
|+    }
|+    visit_type_Abort_fields(v, obj, &err);
|+    error_propagate(errp, err);
|+    err = NULL;
|+out_obj:
|+    visit_end_struct(v, &err);
|+out:
|     error_propagate(errp, err);
| }

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1454075341-13658-12-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -2,7 +2,7 @@
 # QAPI visitor generator
 #
 # Copyright IBM, Corp. 2011
-# Copyright (C) 2014-2015 Red Hat, Inc.
+# Copyright (C) 2014-2016 Red Hat, Inc.
 #
 # Authors:
 #  Anthony Liguori <aliguori@us.ibm.com>
@@ -123,12 +123,18 @@ def gen_visit_struct(name, base, members):
     Error *err = NULL;
 
     visit_start_struct(v, (void **)obj, "%(name)s", name, sizeof(%(c_name)s), &err);
-    if (!err) {
-        if (*obj) {
-            visit_type_%(c_name)s_fields(v, obj, errp);
-        }
-        visit_end_struct(v, &err);
+    if (err) {
+        goto out;
+    }
+    if (!*obj) {
+        goto out_obj;
     }
+    visit_type_%(c_name)s_fields(v, obj, &err);
+    error_propagate(errp, err);
+    err = NULL;
+out_obj:
+    visit_end_struct(v, &err);
+out:
     error_propagate(errp, err);
 }
 ''',