app: fix creating dom0 object when not already present in qubes.xml

It's constant properties are now really constant, no need to provide
them explicitly.

qubes/app.py

@@ -837,7 +837,7 @@ class Qubes(qubes.PropertyHolder):
 
         if 0 not in self.domains:
             self.domains.add(
-                qubes.vm.adminvm.AdminVM(self, None, qid=0, name='dom0'),
+                qubes.vm.adminvm.AdminVM(self, None),
                 _enable_events=False)
 
         # stage 3: load global properties

qubes/vm/__init__.py

@@ -354,6 +354,9 @@ class BaseVM(qubes.PropertyHolder):
         del self.tags
 
     def load_extras(self):
+        if self.xml is None:
+            return
+
         # features
         for node in self.xml.xpath('./features/feature'):
             self.features[node.get('name')] = node.text