vm.backup_xml() is not a sugguested way to get a backup for vm, because it is using /tmp and it will hide some error by excepting Exception in it. Currently, we are using the structure as below.
```
vmxml_backup=vm_xml.VMXML.new_from_inactive_dumpxml(vm_name)
try:
    try:
    ... 
    except:
finally:
    vmxml_backup.sync()
```