only attemp to backup existing xml files

slapos/format.py

@@ -289,6 +289,7 @@ class Computer(object):
           # computer configuration did not change, nothing to write
           return
 
+    if os.path.exists(path_to_xml):
       self.backup_xml(path_to_archive, path_to_xml)
 
     with open(path_to_xml, 'wb') as fout: