Comment 11 for bug 1808089

Reviewed: https://review.openstack.org/633960
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=e7dc43bf848105fdd24b89216676e3a56f1f6689
Submitter: Zuul
Branch: stable/rocky

commit e7dc43bf848105fdd24b89216676e3a56f1f6689
Author: Kevin_Zheng <email address hidden>
Date: Thu Dec 13 19:31:13 2018 +0800

    Don't call begin_detaching when detaching volume from shelved vm

    When shelve an instance, if the instance has volume attached,
    with new attach/detach flow, we will delete the old attachment
    and create a new attachment, the volume status will be ``reserved``.

    If the user tries to detach these volumes, it fails due to that
    Cinder does not allow a begin_detaching() call on a `reserved` volume.

    Actually for shelved instances, we can just skip this step and
    directly detach it.

    Change-Id: Ib1799feebbd8f4b0f389168939df7e5e90c8add1
    closes-bug: #1808089
    (cherry picked from commit 41b982c9feec3105247bc72d23d1470bcabc3a0f)