hw/arm/armv7: Fix crash when introspecting the "iotkit" device

hw/arm/armv7: Fix crash when introspecting the "iotkit" device

QEMU currently crashes when introspecting the "iotkit" device and
runnint "info qtree" afterwards, e.g. when running QEMU like this:

echo "{'execute':'qmp_capabilities'} {'execute':'device-list-properties'," \
 "'arguments':{'typename':'iotkit'}}" "{'execute': 'human-monitor-command', " \
 "'arguments': {'command-line': 'info qtree'}}" | \
 aarch64-softmmu/qemu-system-aarch64 -M none,accel=qtest -qmp stdio

Use the new functions object_initialize_child() and sysbus_init_child_obj()
to make sure that all objects get cleaned up correctly when the instances
are 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>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1531745974-17187-5-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>