core: make sure that dom0.libvirt_domain isn't used

libvirt do not have domain object for dom0, so do not try to access it.

core-modules/006QubesAdminVm.py

@@ -57,7 +57,7 @@ class QubesAdminVm(QubesNetVm):
 
     @property
     def libvirt_domain(self):
-        return None
+        raise ValueError("Dom0 do not have libvirt object")
 
     @property
     def type(self):