condition for setting default value was wrong

This caused an error:
Error occured while creating new network: Call to virNetworkDefineXML failed: (network_definition):5: Unescaped '<' not allowed in attributes values
  <forward mode="#<Object:0x000000020bb1a0>" />
Merge pull request #243 from dennisklein/management_network_mode_typo

condition for setting default value was wrong