Fix dangling LUN issue under load with multipath

This fixes an issue where not all of the LUNs are seen
by the kernel at attach time, but later become available.
We now rescan the list of devices seen by multipath at
detach time.

Also added another unit test case.

Fixes Bug: #1175366

Change-Id: Id5b313b17454ec32672373b7b564b9450466b7a2
Merge "Fix dangling LUN issue under load with multipath"