libvirt: use six.text_type when setting text node value in guest xml

Trying to spawn an instance with a unicode name using the libvirt driver
fails with a UnicodeDecodeError because the value is cast to str().

The fix is to use six.text_type for the cast.

Closes-Bug: #1382318

Change-Id: I4628b94459a3c1e757d388916f1268884cb02038
Merge "libvirt: use six.text_type when setting text node value in guest xml"