qom: Implement info qom-tree HMP command

To complement qdev's bus-oriented info qtree, info qom-tree
prints a hierarchical view of the QOM composition tree.

By default, the machine composition tree is shown. This can be overriden
by supplying a path argument, such as "info qom-tree /".

Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

monitor.c

@@ -2889,6 +2889,13 @@ static mon_cmd_t info_cmds[] = {
         .help       = "show qdev device model list",
         .mhandler.cmd = hmp_info_qdm,
     },
+    {
+        .name       = "qom-tree",
+        .args_type  = "path:s?",
+        .params     = "[path]",
+        .help       = "show QOM composition tree",
+        .mhandler.cmd = hmp_info_qom_tree,
+    },
     {
         .name       = "roms",
         .args_type  = "",