hw/arm/stm32f205_soc: Fix introspection problem with 'stm32f205-soc' device

hw/arm/stm32f205_soc: Fix introspection problem with 'stm32f205-soc' device

Valgrind complains:

echo "{'execute':'qmp_capabilities'} {'execute':'device-list-properties'," \
 "'arguments':{'typename':'stm32f205-soc'}}" \
 "{'execute': 'human-monitor-command', " \
 "'arguments': {'command-line': 'info qtree'}}" | \
 valgrind -q aarch64-softmmu/qemu-system-aarch64 -M none,accel=qtest -qmp stdio
[...]
==28531== Invalid read of size 8
==28531==    at 0x6185BA: qdev_print (qdev-monitor.c:686)
==28531==    by 0x6185BA: qbus_print (qdev-monitor.c:719)
==28531==    by 0x452B38: handle_hmp_command (monitor.c:3446)
[...]

Fix it with the new sysbus_init_child_obj() function.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1531745974-17187-16-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>