Fix XML UnicodeEncode serialization error

The generic Nova XMLSerializer code will currently attempt
to cast to str the value for all leaf nodes. This patch
ensures that no attempt is made to convert unicode which
can cause a UnicodeEncode error. We don't need to convert
unicode for XML text and regardless we encode to UTF-8 at
a later point.

Change-Id: I8135d2b9a67db62b0eafdd301b7fdb67a5dd72cc
Closes-Bug: #1279172
(cherry picked from commit 53fe8696314fb73ca9943fce998d96fa6d0414b4)