Port xio driver to Python 3

Port xio driver to Python 3

* Replace base64.encodestring() with
  oslo_serialization.base64.encode_as_text() to get Unicode
  on Python 3.
* Replace string.upper(text) with text.upper()
* Replace string.replace(text, a, b) with text.replace(a, b)
* tox.ini: add test_xio to Python 3.4

Partial-Implements: blueprint cinder-python3
Change-Id: I75a5dee2fd1f8cf45dbf416b0353736a43efd8e8