tests/migration-test: Fix read off end of aarch64_kernel array

tests/migration-test: Fix read off end of aarch64_kernel array

The test aarch64 kernel is in an array defined with
 unsigned char aarch64_kernel[] = { [...] }

which means it could be any size; currently it's quite small.
However we write it to a file using init_bootfile(), which
writes exactly 512 bytes to the file. This will break if
we ever end up with a kernel larger than that, and will
read garbage off the end of the array in the current setup
where the kernel is smaller.

Make init_bootfile() take an argument giving the length of
the data to write. This allows us to use it for all architectures
(previously s390 had a special-purpose init_bootfile_s390x
which hardcoded the file to write so it could write the
correct length). We assert that the x86 bootfile really is
exactly 512 bytes as it should be (and as we were previously
just assuming it was).

This was detected by the clang-7 asan:
==15607==ERROR: AddressSanitizer: global-buffer-overflow on address 0x55a796f51d20 at pc 0x55a796b89c2f bp 0x7ffc58e89160 sp 0x7ffc58e88908
READ of size 512 at 0x55a796f51d20 thread T0
    #0 0x55a796b89c2e in fwrite (/home/petmay01/linaro/qemu-from-laptop/qemu/build/sanitizers/tests/migration-test+0xb0c2e)
    #1 0x55a796c46492 in init_bootfile /home/petmay01/linaro/qemu-from-laptop/qemu/tests/migration-test.c:99:5
    #2 0x55a796c46492 in test_migrate_start /home/petmay01/linaro/qemu-from-laptop/qemu/tests/migration-test.c:593
    #3 0x55a796c44101 in test_baddest /home/petmay01/linaro/qemu-from-laptop/qemu/tests/migration-test.c:854:9
    #4 0x7f906ffd3cc9  (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x72cc9)
    #5 0x7f906ffd3bfa  (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x72bfa)
    #6 0x7f906ffd3bfa  (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x72bfa)
    #7 0x7f906ffd3ea1 in g_test_run_suite (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x72ea1)
    #8 0x7f906ffd3ec0 in g_test_run (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x72ec0)
    #9 0x55a796c43707 in main /home/petmay01/linaro/qemu-from-laptop/qemu/tests/migration-test.c:1187:11
    #10 0x7f906e9abb96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #11 0x55a796b6c2d9 in _start (/home/petmay01/linaro/qemu-from-laptop/qemu/build/sanitizers/tests/migration-test+0x932d9)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190702150311.20467-1-peter.maydell@linaro.org