iscsi volume revert snapshot failed

[youyk]

This issue looks like reverting 2 snapshots to same volume will fail.

2015-06-12 17:28:00,270 WARN VolumeSnapshotTreeBase failed to restore volume[uuid:1047d20a70fc4567b2f614ec81edb7f7] to snapshot[uuid:4bab7dfa69fd445cba53840444301c53, name:create_snapshot1], ErrorCode [code = SYS.1006, description = An operation failed, details = subvolume[/home/btrfs/rootVolumes/acct-36c27e8ff05c4780bf6d2fa65700f22e/vol-by-snapshot-1047d20a70fc4567b2f614ec81edb7f7] existing]
2015-06-12 17:28:00,270 TRACE CloudBusImpl2 [event publish]: {"org.zstack.header.storage.snapshot.APIRevertVolumeFromSnapshotEvent":{"apiId":"ed2d472c505e42c8ab51242742d15419","success":false,"error":{"code":"SYS.1006","description":"An operation failed","details":"subvolume[/home/btrfs/rootVolumes/acct-36c27e8ff05c4780bf6d2fa65700f22e/vol-by-snapshot-1047d20a70fc4567b2f614ec81edb7f7] existing"},"headers":{"schema":{}},"id":"adc22cb23ebf4b259439e23abd98a100","creatingTime":1434101280253}}
2015-06-12 17:28:00,270 TRACE CloudBusImpl2 [event received]: {"org.zstack.header.storage.snapshot.APIRevertVolumeFromSnapshotEvent":{"type":{"_name":"key.event.API.API_EVENT"},"apiId":"ed2d472c505e42c8ab51242742d15419","success":false,"error":{"code":"SYS.1006","description":"An operation failed","details":"subvolume[/home/btrfs/rootVolumes/acct-36c27e8ff05c4780bf6d2fa65700f22e/vol-by-snapshot-1047d20a70fc4567b2f614ec81edb7f7] existing"},"headers":{"schema":{}},"id":"adc22cb23ebf4b259439e23abd98a100","creatingTime":1434101280253}}

[zxwing]

cbc751c