libvirt_xml and xml_utils unittests: Fix tools/run_unittests.py

Our unittest runner imports the unittest modules and
runs them. Due to the fact that currently our xml modules
rely on the fact they will ever be imported on a scope
where they are visible, this causes the tests executed
under the test runner to fail, even when they pass fine
executed stand alone.

To make the matters worse, we ship a fork of ElementTree,
because we have to support python 2.4. Oh, the joys of
supporting RHEL5...

So we have to handle the different import scenarios
in a way where both the stand alone tests and running
under the test runner pass. Specifically, we *must*
import our specific ElementTree fork.

With this patch, all unit tests run fine both stand
alone and with the test runner. I'm not sure if this
introduces some subtle problems in the running code,
so I kindly ask Chris or Yu to take a look and run the
actual test code just to double check if this is OK.

CC: Yu Mingfei <yumingfei@cn.fujitsu.com>
CC: Chris Evich <cevich@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>