Only reverify backing lun when create backing lun
Only reverify backing lun when create backing lun

In TgtAdm, we alway do twice _verify_backing_lun whether backing lun
exists or not.
If backing lun exists, it doesn't need to call _verify_backing_lun
function again.
It only needs reverify whether backing lun exists or not when recreate
backing lun.

Change-Id: Ic11d778362540df307056a1adcba932e571b7d20
Closes-Bug: #1262958

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