Comment 7 for bug 1800511

Reviewed: https://review.openstack.org/614008
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=643f53f5e9544d6c98833b1ae3dd472602118a1f
Submitter: Zuul
Branch: master

commit 643f53f5e9544d6c98833b1ae3dd472602118a1f
Author: Mohammed Naser <email address hidden>
Date: Mon Oct 29 19:49:41 2018 +0100

    libvirt: Avoid setting MTU during live migration if unset

    If there is a live migration of an instance that was launched
    before change Iecc265fb25e88fa00a66f1fd38e215cad53e7669, it
    would not have an mtu set and therefore it wouldn't have it in
    the XML.

    When live migrating, the mtu is added which changes the
    guest ABI[1], causing the live migration to fail. The failure
    occurs when trying to live migrate an instance that:

    - Launched before change Iecc265fb25e88fa00a66f1fd38e215cad53e7669
    - It has not been rebooted (i.e. XML has not changed since)
    - It's using bridge/tap networking
    - Migration attempted after change Iecc265fb25e88fa00a66f1fd38e215cad53e7669

    This patch prevents this by avoiding setting MTU if the running
    instance does not have one configured in its domain XML.

    [1]: https://bugzilla.redhat.com/show_bug.cgi?id=1449346

    Closes-Bug #1800511
    Change-Id: I6e2e6437a7c826dc425d8b353c38670d6eece0b5