kunit: tool: Use qemu-system-i386 for i386 runs

We're currently using the x86_64 qemu for i386 builds. While this is not
incorrect, it's probably more sensible to use the i386 one, which will
at least fail properly if we accidentally were to build a 64-bit kernel.

Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>