Snapshots and migrations were coded with a simplifying assumption that
the maximum length of a VDI chain would be 3. Now that fast-cloning has
been added, this assumption no longer holds.
The goal of this patch is to remove the restriction for snapshots. A
follow-on patch will remove the restriction for migrations.
This patch changes the image-format for XenAPI images. Instead of naming
the VHDs, 'base', 'image', and 'snap', they are now numbered staring
with 0 as the leaf and going to N as the base-copy (root).
Old-style images are still supported.
Change-Id: Ieb073b42dc25db7cee4dfca7ff6525f7e7f46e8e
08d932659d