Fix disconnecting necessary iSCSI sessions issue

In Icehouse with "iscsi_use_multipath=true", detaching a multipath
iSCSI volume kills all iSCSI volumes visible from the nova compute node.
When we use different targets(IQNs) associated with same portal for
each different multipath device, all of the targets will be deleted
via disconnect_volume().

This patch fixes the behavior of detaching volume:

  1. Extract the targets for the detached multipath device.
  2. Delete/disconnect the targets for the detached multipath device.

Closes-Bug: #1382440

Change-Id: I38eafdaee03d136282cfde1fd013e322a4256cc4
Merge "Fix disconnecting necessary iSCSI sessions issue"