qom: qom_{get, set} monitor commands (v2)

This allows clients to read and write device model properties through QMP.  QAPI
doesn't support Visitor types yet and these commands are special in that they
don't work with fixed types.

I've added a documentation stub to qapi-schema.json so we can keep consistency
there.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/monitor.h b/monitor.h
index 052f1cb..cfa2f67 100644
--- a/monitor.h
+++ b/monitor.h
@@ -67,4 +67,8 @@
 
 void monitor_set_error(Monitor *mon, QError *qerror);
 
+int qmp_qom_set(Monitor *mon, const QDict *qdict, QObject **ret);
+
+int qmp_qom_get(Monitor *mon, const QDict *qdict, QObject **ret);
+
 #endif /* !MONITOR_H */