qmp: fix object-add assert() without props

Since commit ad739706, user_creatable_add_type() expects to be
given a qdict. However, if object-add is called without props, you reach
the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
Assertion `qdict' failed.", because the qdict isn't created in this
case (it's optional).

Furthermore, qmp_input_visitor_new() is not meant to be called without a
dict, and a further commit will assert in this situation.

If none given, create an empty qdict in qmp to avoid the
user_creatable_add_type() assert(qdict).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20160922203927.28241-2-marcandre.lureau@redhat.com>
Tested-by: Xiao Long Jiang <zxiaol@linux.vnet.ibm.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qmp.c

@@ -660,7 +660,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
 void qmp_object_add(const char *type, const char *id,
                     bool has_props, QObject *props, Error **errp)
 {
-    const QDict *pdict = NULL;
+    QDict *pdict;
     Visitor *v;
     Object *obj;
 
@@ -670,14 +670,18 @@ void qmp_object_add(const char *type, const char *id,
             error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
             return;
         }
+        QINCREF(pdict);
+    } else {
+        pdict = qdict_new();
     }
 
-    v = qmp_input_visitor_new(props, true);
+    v = qmp_input_visitor_new(QOBJECT(pdict), true);
     obj = user_creatable_add_type(type, id, pdict, v, errp);
     visit_free(v);
     if (obj) {
         object_unref(obj);
     }
+    QDECREF(pdict);
 }
 
 void qmp_object_del(const char *id, Error **errp)