Code review comment for ~powersj/cloud-init:cii-fix-kvm-tests

tmpfile should look more like this:

    def tmpfile(self):
        path = "/tmp/%s-%04d" % (type(self).__name__, self._tmp_count)
        self._tmp_count += 1
        return path

then we're good.
as is, the tmpfile name never changed, which is kind of not very tmpfile-like