_wait_for_coalesce can hang forever

[Rick Harris]

_wait_for_coalesce is designed to wait for the VDI chain to stabilize so we can upload snapshots of the instance.

If the original VDI doesn't have a parent listed, then this can cause the _wait_for_coalesce to poll forever because the child VHD will never take on a parent of 'None'.

The solution is to only call _wait_for_coalesce if `original_parent_uuid` is not None.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/36924

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/36924
Committed: http://github.com/openstack/nova/commit/be019af1e9f1fc908d740122ae518505a587a114
Submitter: Jenkins
Branch: master

commit be019af1e9f1fc908d740122ae518505a587a114
Author: Rick Harris <email address hidden>
Date: Sat Jul 13 00:07:54 2013 +0000

    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