qom: Implement qom-list HMP command

Implement it as a wrapper for QMP qom-list, but mimic the behavior of
scripts/qmp/qom-list in making the path argument optional and listing
the root if absent, to hint users what kind of path to pass.

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

hmp-commands.hx

@@ -1671,6 +1671,19 @@ ETEXI
 STEXI
 @item cpu-add @var{id}
 Add CPU with id @var{id}
+ETEXI
+
+    {
+        .name       = "qom-list",
+        .args_type  = "path:s?",
+        .params     = "path",
+        .help       = "list QOM properties",
+        .mhandler.cmd  = hmp_qom_list,
+    },
+
+STEXI
+@item qom-list [@var{path}]
+Print QOM properties of object at location @var{path}
 ETEXI
 
     {