qapi: modify visitor code generation for list iteration

Modify logic such that we never assign values to the list head argument
to progress through the list on subsequent iterations, instead rely only
on having our return value passed back in as an argument on the next
call. Also update QMP I/O visitors and test cases accordingly, and add a
missing test case for QmpOutputVisitor.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qapi/qmp-output-visitor.c

@@ -20,6 +20,7 @@
 typedef struct QStackEntry
 {
     QObject *value;
+    bool is_list_head;
     QTAILQ_ENTRY(QStackEntry) node;
 } QStackEntry;
 
@@ -45,6 +46,9 @@ static void qmp_output_push_obj(QmpOutputVisitor *qov, QObject *value)
     QStackEntry *e = g_malloc0(sizeof(*e));
 
     e->value = value;
+    if (qobject_type(e->value) == QTYPE_QLIST) {
+        e->is_list_head = true;
+    }
     QTAILQ_INSERT_HEAD(&qov->stack, e, node);
 }
 
@@ -122,12 +126,20 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
     qmp_output_push(qov, list);
 }
 
-static GenericList *qmp_output_next_list(Visitor *v, GenericList **list,
+static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp,
                                          Error **errp)
 {
-    GenericList *retval = *list;
-    *list = retval->next;
-    return retval;
+    GenericList *list = *listp;
+    QmpOutputVisitor *qov = to_qov(v);
+    QStackEntry *e = QTAILQ_FIRST(&qov->stack);
+
+    assert(e);
+    if (e->is_list_head) {
+        e->is_list_head = false;
+        return list;
+    }
+
+    return list ? list->next : NULL;
 }
 
 static void qmp_output_end_list(Visitor *v, Error **errp)