Fixes unicode issue in the Hyper-V driver

The XML string returned by the WMI API calls requires unicode
conversion before further processing, e.g. with ElementTree.

This affects the vhdutilsv2._get_vhd_info_xml method due to
the textual description. The issue came to light using the
host with Chinese locale, while not present on a en_US host.

Fixes bug: #1229671

Change-Id: I09435a205abfc8ae9470c53c5446cbb28dee0115
(cherry picked from commit c8c7c01d68b34b76ebe6c4b39f93915480c1ccea)
Merge "Fixes unicode issue in the Hyper-V driver" into stable/havana