Fix corner cases where register's RootDeviceName is unset
Fix corner cases where register's RootDeviceName is unset

Fixes TOOLS-330

jhajek/euca2ools,vasiliykochergin/euca2ools,jhajek/euca2ools,gholms/euca2ools,nagyistoce/euca2ools,nagyistoce/euca2ools,gholms/euca2ools,vasiliykochergin/euca2ools