tests/qtest/fdc-test: Add a regression test for CVE-2021-3507

Add the reproducer from https://gitlab.com/qemu-project/qemu/-/issues/339



Without the previous commit, when running 'make check-qtest-i386'
with QEMU configured with '--enable-sanitizers' we get:

  ==4028352==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x619000062a00 at pc 0x5626d03c491a bp 0x7ffdb4199410 sp 0x7ffdb4198bc0
  READ of size 786432 at 0x619000062a00 thread T0
      #0 0x5626d03c4919 in __asan_memcpy (qemu-system-i386+0x1e65919)
      #1 0x5626d1c023cc in flatview_write_continue softmmu/physmem.c:2787:13
      #2 0x5626d1bf0c0f in flatview_write softmmu/physmem.c:2822:14
      #3 0x5626d1bf0798 in address_space_write softmmu/physmem.c:2914:18
      #4 0x5626d1bf0f37 in address_space_rw softmmu/physmem.c:2924:16
      #5 0x5626d1bf14c8 in cpu_physical_memory_rw softmmu/physmem.c:2933:5
      #6 0x5626d0bd5649 in cpu_physical_memory_write include/exec/cpu-common.h:82:5
      #7 0x5626d0bd0a07 in i8257_dma_write_memory hw/dma/i8257.c:452:9
      #8 0x5626d09f825d in fdctrl_transfer_handler hw/block/fdc.c:1616:13
      #9 0x5626d0a048b4 in fdctrl_start_transfer hw/block/fdc.c:1539:13
      #10 0x5626d09f4c3e in fdctrl_write_data hw/block/fdc.c:2266:13
      #11 0x5626d09f22f7 in fdctrl_write hw/block/fdc.c:829:9
      #12 0x5626d1c20bc5 in portio_write softmmu/ioport.c:207:17

  0x619000062a00 is located 0 bytes to the right of 512-byte region [0x619000062800,0x619000062a00)
  allocated by thread T0 here:
      #0 0x5626d03c66ec in posix_memalign (qemu-system-i386+0x1e676ec)
      #1 0x5626d2b988d4 in qemu_try_memalign util/oslib-posix.c:210:11
      #2 0x5626d2b98b0c in qemu_memalign util/oslib-posix.c:226:27
      #3 0x5626d09fbaf0 in fdctrl_realize_common hw/block/fdc.c:2341:20
      #4 0x5626d0a150ed in isabus_fdc_realize hw/block/fdc-isa.c:113:5
      #5 0x5626d2367935 in device_set_realized hw/core/qdev.c:531:13

  SUMMARY: AddressSanitizer: heap-buffer-overflow (qemu-system-i386+0x1e65919) in __asan_memcpy
  Shadow bytes around the buggy address:
    0x0c32800044f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    0x0c3280004510: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    0x0c3280004520: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    0x0c3280004530: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  =>0x0c3280004540:[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004550: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004560: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004570: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004580: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
    0x0c3280004590: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  Shadow byte legend (one shadow byte represents 8 application bytes):
    Addressable:           00
    Heap left redzone:       fa
    Freed heap region:       fd
  ==4028352==ABORTING

[ kwolf: Added snapshot=on to prevent write file lock failure ]

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>