When running device-introspect-test, a memory leak occurred in the
digic_timer_init function, so use ptimer_free() in the finalize function to
avoid it.

ASAN shows memory leak stack:

Indirect leak of 288 byte(s) in 3 object(s) allocated from:
    #0 0xffffab97e1f0 in __interceptor_calloc (/lib64/libasan.so.5+0xee1f0)
    #1 0xffffab256800 in g_malloc0 (/lib64/libglib-2.0.so.0+0x56800)
    #2 0xaaabf555db78 in ptimer_init /qemu/hw/core/ptimer.c:432
    #3 0xaaabf5b04084 in digic_timer_init /qemu/hw/timer/digic-timer.c:142
    #4 0xaaabf6339f6c in object_initialize_with_type /qemu/qom/object.c:515
    #5 0xaaabf633ca04 in object_initialize_child_with_propsv /qemu/qom/object.c:564
    #6 0xaaabf633cc08 in object_initialize_child_with_props /qemu/qom/object.c:547
    #7 0xaaabf5b40e84 in digic_init /qemu/hw/arm/digic.c:46
    #8 0xaaabf6339f6c in object_initialize_with_type /qemu/qom/object.c:515
    #9 0xaaabf633a1e0 in object_new_with_type /qemu/qom/object.c:729
    #10 0xaaabf6375e40 in qmp_device_list_properties /qemu/qom/qom-qmp-cmds.c:153
    #11 0xaaabf653d8ec in qmp_marshal_device_list_properties /qemu/qapi/qapi-commands-qdev.c:59
    #12 0xaaabf6587d08 in do_qmp_dispatch_bh /qemu/qapi/qmp-dispatch.c:110

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>