Allow Domain class to be specified by Connection class

This allows subclasses of Connection to use a subclass of Domain
without any fuss.

master/buildbot/libvirtbuildslave.py

@@ -109,6 +109,8 @@ class Connection(object):
     I am a wrapper around a libvirt Connection object.
     """
 
+    DomainClass = Domain
+
     def __init__(self, uri):
         self.uri = uri
         self.connection = libvirt.open(uri)
@@ -117,15 +119,15 @@ def lookupByName(self, name):
         """ I lookup an existing prefined domain """
         d = queue.executeInThread(self.connection.lookupByName, name)
         def _(res):
-            return Domain(self, res)
+            return self.DomainClass(self, res)
         d.addCallback(_)
         return d
 
     def create(self, xml):
         """ I take libvirt XML and start a new VM """
         d = queue.executeInThread(self.connection.createXML, xml, 0)
         def _(res):
-            return Domain(self, res)
+            return self.DomainClass(self, res)
         d.addCallback(_)
         return d