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[RBD block device ]ignore the loopbackdevice error, or the rbd volume will not get detached

The rbd volume loop device maybe not found and then the volume will  not get detached.
so need to ignore the error.
@mtanino @sbezverk
Can you take a look?

Release note:
```
ignore the loopback device not found error when  `GetLoopDevice` 
```