qom: Implement qom-list HMP command

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>