hmp: Use QAPI NetdevInfo in hmp_info_network

Replace usage of legacy field info_str of NetClientState for backend
network devices with QAPI NetdevInfo stored_config that already used
in QMP query-netdev.

This change increases the detail of the "info network" output and takes
a more general approach to composing the output.

NIC and hubports still use legacy info_str field.

Signed-off-by: Alexey Kirillov <lekiravi@yandex-team.ru>
Signed-off-by: Jason Wang <jasowang@redhat.com>

include/qapi/hmp-output-visitor.h

@@ -0,0 +1,30 @@
+/*
+ * HMP string output Visitor
+ *
+ * Copyright Yandex N.V., 2021
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef HMP_OUTPUT_VISITOR_H
+#define HMP_OUTPUT_VISITOR_H
+
+#include "qapi/visitor.h"
+
+typedef struct HMPOutputVisitor HMPOutputVisitor;
+
+/**
+ * Create a HMP string output visitor for @obj
+ *
+ * Flattens dicts/structures, only shows arrays borders.
+ *
+ * Errors are not expected to happen.
+ *
+ * The caller is responsible for freeing the visitor with
+ * visit_free().
+ */
+Visitor *hmp_output_visitor_new(char **result);
+
+#endif