libvirt: Regenerates xml instead of using on-disk

The libvirt.xml file is written on initial boot of the instance and
may not reflect the current state of the vm if volumes have been
attached. If the libvirt definition of the vm has been lost, it is
safer to regenerate it rather than attempt to load it from the
outdated file. This should properly bring back vms with attached
volumes.

Fixes bug 1073720

Change-Id: Iaa754700a149f09fc0c022fa664c06ad17f505f5
(cherry picked from commit b22c330)

nova/virt/libvirt/driver.py

@@ -678,15 +678,13 @@ def _get_disk_xml(xml, device):
                     if child.get('dev') == device:
                         return etree.tostring(node)
 
-    def _get_domain_xml(self, instance):
+    def _get_domain_xml(self, instance, network_info, block_device_info=None):
         try:
             virt_dom = self._lookup_by_name(instance['name'])
             xml = virt_dom.XMLDesc(0)
         except exception.InstanceNotFound:
-            instance_dir = os.path.join(FLAGS.instances_path,
-                                        instance['name'])
-            xml_path = os.path.join(instance_dir, 'libvirt.xml')
-            xml = libvirt_utils.load_file(xml_path)
+            xml = self.to_xml(instance, network_info,
+                              block_device_info=block_device_info)
         return xml
 
     @exception.wrap_exception()
@@ -929,7 +927,8 @@ def _hard_reboot(self, instance, network_info, xml=None,
         """
 
         if not xml:
-            xml = self._get_domain_xml(instance)
+            xml = self._get_domain_xml(instance, network_info,
+                                       block_device_info)
 
         self._destroy(instance)
         self._create_domain_and_network(xml, instance, network_info,
@@ -988,7 +987,7 @@ def resume(self, instance):
     def resume_state_on_host_boot(self, context, instance, network_info,
                                   block_device_info=None):
         """resume guest state when a host is booted"""
-        xml = self._get_domain_xml(instance)
+        xml = self._get_domain_xml(instance, network_info, block_device_info)
         self._create_domain_and_network(xml, instance, network_info,
                                         block_device_info)
 
@@ -1004,7 +1003,7 @@ def rescue(self, context, instance, network_info, image_meta,
 
         """
 
-        unrescue_xml = self._get_domain_xml(instance)
+        unrescue_xml = self._get_domain_xml(instance, network_info)
         unrescue_xml_path = os.path.join(FLAGS.instances_path,
                                          instance['name'],
                                          'unrescue.xml')