Fixing LeftHand live migration error

Fixed a bug in the LeftHand driver that was causing an error to occur during
live migration of an instance that is attached to a volume. The solution was
to make sure server access is only requested for a volume if the iSCSI
session doesn't already have access enabled on the LeftHand backend.

Change-Id: I41046beb66f2a6138659ec3e4d1d3f37b37859b8
Closes-Bug: 1311350
Merge "Fixing LeftHand live migration error"