hw/arm/fsl-imx31: Fix introspection problem with the "fsl, imx31" device

hw/arm/fsl-imx31: Fix introspection problem with the "fsl, imx31" device

Running QEMU with valgrind indicates a problem here:

echo "{'execute':'qmp_capabilities'} {'execute':'device-list-properties'," \
 "'arguments':{'typename':'fsl,imx31'}}" \
 "{'execute': 'human-monitor-command', " \
 "'arguments': {'command-line': 'info qtree'}}" | \
 valgrind -q aarch64-softmmu/qemu-system-aarch64 -M none,accel=qtest -qmp stdio
[...]
==26172== Invalid read of size 8
==26172==    at 0x6191FA: qdev_print (qdev-monitor.c:686)
==26172==    by 0x6191FA: qbus_print (qdev-monitor.c:719)
[...]

Use the new sysbus_init_child_obj() to make sure that the objects are
cleaned up correctly when the parent gets destroyed.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1531745974-17187-12-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>