ゲストOSのブリッジ設定でのエラー
# service network start Bringing up loopback interface: [ OK ] Bringing up interface eth0: Error: Connection activation failed: Master connection not found or invalid [FAILED] Bringing up interface br0: Error: Connection activation failed: Failed to determine connection's virtual interface name [FAILED]
こういう場合はNW_CONTROLLED="no"にすればうまくいくみたい。