Support efficient non-disruptive volume backup in VNX

Support efficient non-disruptive volume backup in VNX

Currently, VNX only supports creating backup for in-use volume via cloned volume.
Due to the slow lun migration in creating cloned volume,
A better solution is proposed below:
https://review.openstack.org/#/c/201249/

This patch is to implement above feature in the VNX cinder driver.

Change-Id: Ic0765081a88f28d3114f28b50c4f0a1b6d7ef9dd
Implements: blueprint non-disruptive-backup-in-vnx