qubes/storage: Fix template instantiation

rootcow image was somehow dropped

qubes/storage/__init__.py

@@ -79,6 +79,12 @@ class VMStorage(object):
             else self.abspath(qubes.config.vm_files['root_img'])
 
 
+    @property
+    def rootcow_img(self):
+        '''Path to the root COW image'''
+        return self.abspath(qubes.config.vm_files['rootcow_img'])
+
+
     @property
     def volatile_img(self):
         '''Path to the volatile image'''