hw/i386/acpi-build: decref after use

hw/i386/acpi-build: decref after use

valgrind complains about:
==16447== 48 bytes in 2 blocks are definitely lost in loss record 2,033 of 3,310
==16447==    at 0x4C2845D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==16447==    by 0x2E4FD7: malloc_and_trace (vl.c:2546)
==16447==    by 0x64C770E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
==16447==    by 0x53EC3F: qint_from_int (qint.c:33)
==16447==    by 0x53B426: qmp_output_type_int (qmp-output-visitor.c:162)
==16447==    by 0x539257: visit_type_uint32 (qapi-visit-core.c:147)
==16447==    by 0x471D07: property_get_uint32_ptr (object.c:1651)
==16447==    by 0x47000C: object_property_get (object.c:822)
==16447==    by 0x472428: object_property_get_qobject (qom-qobject.c:37)
==16447==    by 0x25701A: build_append_pci_bus_devices (acpi-build.c:520)
==16447==    by 0x25902E: build_ssdt (acpi-build.c:1004)
==16447==    by 0x25A0A8: acpi_build (acpi-build.c:1420)

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>