Windows iSCSI: fix volume clone

Windows iSCSI: fix volume clone

In some environments, volume cloning fails when using the Windows
iSCSI volume driver. This involves creating a new VHD image and
copying the data from the source volume on top.

This can raise an error, caused by the fact that the destination
path exists.

This patch slightly changes the workflow used when cloning an image.
Instead of first creating a new WT disk and then copying the cloned
image data on top, the source image is copied, resized if needed
and then imported as a WT disk.

Change-Id: I07d45fa9324a34ef45ed2aa8051338ee6a0f8a5b
Closes-Bug: #1435865