Comment 7 for bug 1782393

Reviewed: https://review.openstack.org/584962
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=3a022299c13ff6a0a949c05c54fdc7331416a0b0
Submitter: Zuul
Branch: master

commit 3a022299c13ff6a0a949c05c54fdc7331416a0b0
Author: Matt Riedemann <email address hidden>
Date: Mon Jul 23 12:15:44 2018 -0400

    Cache is_bfv check in ResourceTracker

    Whether or not an instance is volume-backed shouldn't
    change for the lifetime of the server, so rather than
    figure that out during every operation (or periodic task)
    for an instance in the resource tracker, get the is_bfv
    value once and cache it. The cache entries are removed
    when the instance is removed from the host (either deleted,
    migrated or shelved offloaded).

    Change-Id: Ic5188d4c5a7e8aa4c79c37e63124fe3a305ef568
    Related-Bug: #1782393