Storwize: Fix iogrp availability check

Do not check if vdisk_count is greater than zero to determine iogrp
availability. This is wrong, and leads to the driver not being able to
use empty iogrps.

Closes-Bug: #1240395
Cherry-pick from https://review.openstack.org/#/c/52031/

Change-Id: I50456a0c618103f919ef0da7fb9e44b77a1d9bff