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>