Return instance of Guest from method write_instance_config

We expect an instance of Guest, so remove the TODO notice and
return it.

Change-Id: I9c55883cca03f7492ba100fd60d9f2e58aa71d95

nova/tests/unit/virt/libvirt/test_host.py

@@ -869,9 +869,24 @@ Active:          8381604 kB
 
     @mock.patch.object(fakelibvirt.virConnect, "defineXML")
     def test_write_instance_config(self, mock_defineXML):
-        xml = "<x><name>foo</name></x>"
-        self.host.write_instance_config(xml)
-        mock_defineXML.assert_called_once_with(xml)
+        fake_dom_xml = """
+                <domain type='kvm'>
+                  <uuid>cef19ce0-0ca2-11df-855d-b19fbce37686</uuid>
+                  <devices>
+                    <disk type='file'>
+                      <source file='filename'/>
+                    </disk>
+                  </devices>
+                </domain>
+            """
+        conn = self.host.get_connection()
+        dom = fakelibvirt.Domain(conn,
+                                 fake_dom_xml,
+                                 False)
+        mock_defineXML.return_value = dom
+        guest = self.host.write_instance_config(fake_dom_xml)
+        mock_defineXML.assert_called_once_with(fake_dom_xml)
+        self.assertIsInstance(guest, libvirt_guest.Guest)
 
     @mock.patch.object(fakelibvirt.virConnect, "nodeDeviceLookupByName")
     def test_device_lookup_by_name(self, mock_nodeDeviceLookupByName):

nova/virt/libvirt/guest.py

@@ -120,14 +120,12 @@ class Guest(object):
         :returns guest.Guest: Guest ready to be launched
         """
         try:
-            # TODO(sahid): Host.write_instance_config should return
-            # an instance of Guest
-            domain = host.write_instance_config(xml)
+            guest = host.write_instance_config(xml)
         except Exception:
             with excutils.save_and_reraise_exception():
-                LOG.error(_LE('Error defining a domain with XML: %s'),
+                LOG.error(_LE('Error defining a guest with XML: %s'),
                           encodeutils.safe_decode(xml))
-        return cls(domain)
+        return guest
 
     def launch(self, pause=False):
         """Starts a created guest.

nova/virt/libvirt/host.py

@@ -855,9 +855,10 @@ class Host(object):
 
         :param xml: XML domain definition of the guest.
 
-        :returns: a virDomain instance
+        :returns: an instance of Guest
         """
-        return self.get_connection().defineXML(xml)
+        domain = self.get_connection().defineXML(xml)
+        return libvirt_guest.Guest(domain)
 
     def device_lookup_by_name(self, name):
         """Lookup a node device by its name.