Implement extend volume for Storwize/SVC.

This implements the extend volume functionality for the Storwize/SVC
driver.  Extending volumes with snapshots is not supported, as this
will require converting the snapshots into full copies, which will eat
up free space.

Change-Id: I5fb8c6967e6f3df1adfef46e05c832a953dd7131
Merge "Implement extend volume for Storwize/SVC."