qemu-iotests: 149: Use "/usr/bin/env python"

Do the same as other scripts, to pick the correct interpreter between
python2 and python3 from the environment.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-id: 1459504593-2692-1-git-send-email-famz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/149

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright (C) 2016 Red Hat, Inc.
 #