iotests: handle TypeError for Python 3 in test 242

iotests: handle TypeError for Python 3 in test 242

The data type for bytes in Python 3 differs from the one in Python 2.
The type cast that is compatible with both versions was applied.

Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reported-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1551197495-24425-1-git-send-email-andrey.shinkevich@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>