Fixes cinder volume from image on Windows

Fixes cinder volume from image on Windows

When creating a volume from an image, after the volume is created,
qemu-img is supposed to copy the downloaded image to the volume.
This fails as the disk is enabled and is not accessible.

Also, Windows does not support using dynamic vhds as iSCSI disks
and as qemu-img cannot create fixed vhd images, there must be an
intermediate conversion before importing the disk.

In order to be able to modify an iSCSI disk, it must be disabled
first.

Closes-Bug: #1300906

Change-Id: I726b23287c5b710227288d6215538f01dfb0f979