hw/arm/xlnx-zynqmp: Fix crash when introspecting the "xlnx, zynqmp" device

hw/arm/xlnx-zynqmp: Fix crash when introspecting the "xlnx, zynqmp" device

QEMU currently crashes when e.g. doing something like this:

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

Use the new object_initialize_child() and sysbus_init_child_obj()
functions to get the refernce counting of the child objects right, so
that they are properly cleaned up 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>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1531745974-17187-18-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>