xenapi: Only coalesce VHDs if needed

If the original parent is None, then we don't need to coalesce the VHD since
there won't be a grandparent for the child to coalesce into. Doing otherwise,
causes us to poll forever as we wait for the child's parent to become `None`.

Fixes bug 1200806

Change-Id: I0cbf0be86ff925b83c0b8fa90f88d4ca304fd2b9
Merge "xenapi: Only coalesce VHDs if needed"