hw/i386/pc_piix: Fix memory leak

valgrind complains about:
==16447== 8 bytes in 1 blocks are definitely lost in loss record 552 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 0x24E622: pc_init1 (pc_piix.c:287)
==16447==    by 0x24E76A: pc_init_pci (pc_piix.c:310)
==16447==    by 0x2E9360: main (vl.c:4226)

==16447== 128 bytes in 1 blocks are definitely lost in loss record 2,569 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 0x25BEB2: kvm_i8259_init (i8259.c:133)
==16447==    by 0x24E1F1: pc_init1 (pc_piix.c:219)
==16447==    by 0x24E76A: pc_init_pci (pc_piix.c:310)
==16447==    by 0x2E9360: main (vl.c:4226)

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>
