hw/misc/macio: Fix device introspection problems in macio devices

Valgrind reports an error when introspecting the macio devices, e.g.:

echo "{'execute':'qmp_capabilities'} {'execute':'device-list-properties'," \
 "'arguments':{'typename':'macio-newworld'}}" \
 "{'execute': 'human-monitor-command', " \
 "'arguments': {'command-line': 'info qtree'}}" | \
 valgrind -q ppc64-softmmu/qemu-system-ppc64 -M none,accel=qtest -qmp stdio
[...]
==30768== Invalid read of size 8
==30768==    at 0x5BC1EA: qdev_print (qdev-monitor.c:686)
==30768==    by 0x5BC1EA: qbus_print (qdev-monitor.c:719)
==30768==    by 0x43E458: handle_hmp_command (monitor.c:3446)
[...]

Use the new function sysbus_init_child_obj() to initialize the objects
here, to get the reference counting of the objects right, so that they
are cleaned up correctly when the parent gets removed.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>