libvirt live-migration fails when network MTU is different

[Tyler Stachecki]

After changing the MTU of an OpenStack network, attempting to live-migrate an existing instance fails with e.g.:
ERROR nova.virt.libvirt.driver [-] [instance: f3807b12-fa7e-4da8-a688-c5507fb66891] Live Migration failure: unsupported configuration: Target network card MTU 9000 does not match source 1500: libvirt.libvirtError: unsupported configuration: Target network card MTU 9000 does not match source 1500

Indeed, the MTU should not change as per:
https://bugzilla.redhat.com/show_bug.cgi?id=1623157

However, it should be the case that libvirt live-migration code should retain the MTU specified in the source domain. If we do that, live-migration still works.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/852365

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Tyler Stachecki <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/nova/+/852365
Reason: As per review discussion, the fact that Nova is not able to adapt the MTU on a running instance is problematic for Neutron and vise-versa. A subsequent PS will be pushed with an improved error message to reference this issue in the future.