hw/ide/pci: Fix memory leak

valgrind complains about:
==16447== 16 bytes in 2 blocks are definitely lost in loss record 1,304 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 0x36FB47: qemu_extend_irqs (irq.c:55)
==16447==    by 0x36FBD3: qemu_allocate_irqs (irq.c:64)
==16447==    by 0x3B4B44: bmdma_init (pci.c:464)
==16447==    by 0x3B547B: pci_piix_init_ports (piix.c:144)
==16447==    by 0x3B55D2: pci_piix_ide_realize (piix.c:164)
==16447==    by 0x3EAEC6: pci_qdev_realize (pci.c:1790)
==16447==    by 0x36C685: device_set_realized (qdev.c:1058)
==16447==    by 0x47179E: property_set_bool (object.c:1514)
==16447==    by 0x470098: object_property_set (object.c:837)

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