avocado.plugins.xunit: Fix incorrect double quoting

The extra double quotes left by mistake in the error
reporting code made the XML invalid. Fix that, and make
sure it stays that way with follow up functional tests.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/plugins/xunit.py

@@ -130,8 +130,8 @@ class XmlResult(object):
 
         :param test: an instance of :class:`avocado.test.Test`.
         """
-        tc = '''\t<testcase classname="{class}" name="{name}" time="{time}">
-\t\t<error type="{type}" message={reason}><![CDATA[{traceback}]]></error>
+        tc = '''\t<testcase classname={class} name={name} time="{time}">
+\t\t<error type={type} message={reason}><![CDATA[{traceback}]]></error>
 \t\t<system-out><![CDATA[{systemout}]]></system-out>
 \t</testcase>'''
         values = {'class': self._escape_attr(test.__class__.__name__),