tests: Fix migration-test build failure for sparc

tests: Fix migration-test build failure for sparc

Even if <linux/kvm.h> seems to exist for all archs on linux, however including
it with __linux__ defined seems to be not working yet as it'll try to include
asm/kvm.h and that can be missing for archs that do not support kvm.

To fix this (instead of any attempt to fix linux headers..), we can mark the
header to be x86_64 only, because it's so far only service for adding the kvm
dirty ring test.

Fixes: 1f546b709d6 ("tests: migration-test: Add dirty ring test")
Reported-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210728214128.206198-1-peterx@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>