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

Conflicts:
        nova/virt/libvirt/config.py

Change-Id: I4628b94459a3c1e757d388916f1268884cb02038
(cherry picked from commit 73fcf4628089dd784889062e916b80d3fc9988a2)
Merge "libvirt: use six.text_type when setting text node value in guest xml" into stable/juno