Storwize/SVC driver detach volume failed

If config two backends on the same storage, but the protocol is
different, one is FC and the other is iSCSI,  that will create two
hosts on the storage.

When detaching the volume which is attached to the iSCSI host, the
driver will get FC host by default, that causes volume status keeps
'detaching'.

Change-Id: Idbdb3861574197549625b4a6b6141197e42857ff
(cherry picked from commit a221a30b32aa94077f352719b2637b51c1809069)
Closes-Bug: #1310559
Merge "Storwize/SVC driver detach volume failed" into stable/icehouse