go-tcmu always try to delete existing device or directories

### version

- 68a667dd549b26a602d4be2a54475e3d76e453a3

### issue

go-tcmu always try to delete existing device or directories before running. Since `OpenTCMUDevice()` [calls `Close()`](https://github.com/coreos/go-tcmu/blob/68a667dd549b26a602d4be2a54475e3d76e453a3/device.go#L66) before opening the device, if there is any exsiting devices or directories, they are always deleted.
