tests: Add xmlns remove_child test case

And tighten up the xmlbuilder xmlns hackery

tests/xmlparse.py

@@ -982,7 +982,10 @@ class XMLParseTest(unittest.TestCase):
         check = self._make_checker(guest.xmlns_qemu.args[1])
         check("value", "bar,baz=wib wob", "testval")
         guest.xmlns_qemu.args.add_new().value = "additional-arg"
+        arg0 = guest.xmlns_qemu.args[0]
         guest.xmlns_qemu.remove_child(guest.xmlns_qemu.args[0])
+        self.assertEqual(arg0.get_xml_config(),
+                "<qemu:arg xmlns:qemu=\"http://libvirt.org/schemas/domain/qemu/1.0\" value=\"-somenewarg\"/>\n")
 
         check = self._make_checker(guest.xmlns_qemu.envs[0])
         check("name", "SOMEENV")

virtinst/xmlbuilder.py

@@ -455,8 +455,8 @@ class _XMLState(object):
         if not parsexml:
             parsexml = "<%s%s/>" % (self._root_name, self._namespace)
         elif self._namespace and "xmlns" not in parsexml:
-            parsexml = parsexml.replace(self._root_name,
-                    self._root_name + self._namespace)
+            parsexml = parsexml.replace("<" + self._root_name,
+                    "<" + self._root_name + self._namespace)
 
         try:
             self.xmlapi = XMLAPI(parsexml)