Storwize/SVC: Use VolumeDriver's copy vol<->image.

Inherit the VolumeDriver copy volume<->image functions. This patch also
creates a new SanDriver class which is protocol-agnostic, and leaves
the SanISCSIDriver class for drivers that wish to use it.

Change-Id: Iad5cb030545ef7f4e01d006f1e63cdeeec713bf6
Merge "Storwize/SVC: Use VolumeDriver's copy vol<->image."