Detach volume fails when using multipath iscsi

Enabled multipath for libvirt iscsi volume driver. Attach volume worked,
however, detach volume failed. It failed because it can't find multipath
device "/dev/mapper/xxxxxxxx" from connection_info and block device
mapping table.

The fix is to retrieve multipath device info dynamically at detach time.

Fixes: bug #1178893

Change-Id: I53a3888daef93b57bf7f57438f74cee05e794b52
(cherry picked from commit fc786dd469a15bda1b9d3c7bacf9f1771b9b9956)
Merge "Detach volume fails when using multipath iscsi" into stable/grizzly