Generic iSCSI copy volume<->image.

Implements a generic version of copy_volume_to_image and
copy_image_to_volume for iSCSI drivers.

Change-Id: Iff097629bcce9154829a7eb5aee0ea6302338b26
Implements: blueprint generic-iscsi-copy-vol-image
Merge "Generic iSCSI copy volume<->image."