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
Closes-Bug: #1310559