fix pre_block_migration() interaction with libvirt cache

Fixes bug 932179

* nova/virt/libvirt/connection (pre_block_migration):
Don't pass the 'ephemeral_gb' attribute as the size,
since this has the wrong units and is the wrong quantity
to pass anyway.  Also don't check for file existence,
or create the cow image as this is done within _cache_image()

Change-Id: Id723ec8d119dacd4660e88e546e05af417c736fc

nova/virt/libvirt/connection.py

@@ -1909,24 +1909,19 @@ def pre_block_migration(self, ctxt, instance_ref, disk_info_json):
                                            info['disk_size'])
             else:
                 # Creating backing file follows same way as spawning instances.
-                backing_file = os.path.join(FLAGS.instances_path,
-                                            '_base', info['backing_file'])
-
+                cache_name = os.path.basename(info['backing_file'])
                 # Remove any size tags which the cache manages
-                cached_file = info['backing_file'].split('_')[0]
-
-                if not os.path.exists(backing_file):
-                    self._cache_image(fn=libvirt_utils.fetch_image,
-                        context=ctxt,
-                        target=info['path'],
-                        fname=cached_file,
-                        cow=FLAGS.use_cow_images,
-                        image_id=instance_ref['image_ref'],
-                        user_id=instance_ref['user_id'],
-                        project_id=instance_ref['project_id'],
-                        size=instance_ref['ephemeral_gb'])
-
-                libvirt_utils.create_cow_image(backing_file, instance_disk)
+                cache_name = cache_name.split('_')[0]
+
+                self._cache_image(fn=libvirt_utils.fetch_image,
+                    context=ctxt,
+                    target=instance_disk,
+                    fname=cache_name,
+                    cow=FLAGS.use_cow_images,
+                    image_id=instance_ref['image_ref'],
+                    user_id=instance_ref['user_id'],
+                    project_id=instance_ref['project_id'],
+                    size=info['disk_size'])
 
         # if image has kernel and ramdisk, just download
         # following normal way.