Fixed a problem in iSCSI multipath

Multipathing for attach volume and detach volume operations
doesn't work properly if there are different targets
associated with different portals for a mulitpath device.

Closes-Bug: #1266051
(cherry picked from commit 429ac4dedd617f8c1f7c88dd8ece6b7d2f2accd0)

Conflicts:
	nova/tests/virt/libvirt/test_libvirt_volume.py
	nova/virt/libvirt/volume.py

Change-Id: Iab316526cf64de60325d90043f73fa3e83fafb4e
Merge "Fixed a problem in iSCSI multipath" into stable/havana