xunit plugin: fix DOM usage

On Python 3, it's not possible to create Elements detached from a
Document.  The following code fails (later than it should) on Python
3:

   >>> from xml.dom.minidom import Element
   >>> e = Element('foo')
   >>> e.setAttribute('bar', 'baz')
   Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "/usr/lib64/python3.6/xml/dom/minidom.py", line 741, in setAttribute
      attr.ownerDocument = self.ownerDocument
   AttributeError: ownerDocument

Let's create elements within the document, with the `createElement`
API.  According to the docs "The element is not inserted into the
document when it is created", so the (unchanged) code that actually puts
the element in the right place still applies.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/plugins/xunit.py

@@ -18,7 +18,7 @@
 import datetime
 import os
 import string
-from xml.dom.minidom import Document, Element
+from xml.dom.minidom import Document
 
 from avocado.core.parser import FileOrStdoutAction
 from avocado.core.output import LOG_UI
@@ -54,13 +54,13 @@ class XUnitResult(Result):
         return testcase
 
     def _create_failure_or_error(self, document, test, element_type):
-        element = Element(element_type)
+        element = document.createElement(element_type)
         element.setAttribute('type', self._get_attr(test, 'fail_class'))
         element.setAttribute('message', self._get_attr(test, 'fail_reason'))
         traceback_content = self._escape_cdata(test.get('traceback', self.UNKNOWN))
         traceback = document.createCDATASection(traceback_content)
         element.appendChild(traceback)
-        system_out = Element('system-out')
+        system_out = document.createElement('system-out')
         try:
             with open(test.get("logfile"), "r") as logfile_obj:
                 text_output = logfile_obj.read()
@@ -88,7 +88,7 @@ class XUnitResult(Result):
             if status in ('PASS', 'WARN'):
                 pass
             elif status == 'SKIP':
-                testcase.appendChild(Element('skipped'))
+                testcase.appendChild(document.createElement('skipped'))
             elif status == 'FAIL':
                 element, system_out = self._create_failure_or_error(document,
                                                                     test,
@@ -96,7 +96,7 @@ class XUnitResult(Result):
                 testcase.appendChild(element)
                 testcase.appendChild(system_out)
             elif status == 'CANCEL':
-                testcase.appendChild(Element('skipped'))
+                testcase.appendChild(document.createElement('skipped'))
             else:
                 element, system_out = self._create_failure_or_error(document,
                                                                     test,