Comment 14 for bug 1796981

Reviewed: https://review.openstack.org/611326
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=940034c27c611b8b344f8371eb0458b16fb58f1d
Submitter: Zuul
Branch: stable/rocky

commit 940034c27c611b8b344f8371eb0458b16fb58f1d
Author: Matt Riedemann <email address hidden>
Date: Wed Oct 10 15:07:00 2018 -0400

    Fix NoneType error in _notify_volume_usage_detach

    If the driver.block_stats() method returns None, like the
    libvirt driver will if the guest is gone during an evacuate,
    we'll get a NoneType error trying to unpack the return value
    from the driver. Instead, simply return as if the driver
    raised NotImplementedError.

    Since handling None is changing the contract on the virt
    driver API, the docstring is updated to explain the acceptable
    return values of the driver method.

    Change-Id: I98a2785c07f7af02ad83650c72d9e1868290ece4
    Closes-Bug: #1796981
    (cherry picked from commit 4da54c07861a1542a8e30a768d4506d3e81b5598)