Description of problem:
VMs cannot live migrate after MTU of a network its port is in is changed.

Version-Release number of selected component (if applicable):
openstack-nova-compute-20.4.1-1.20200917173450.el8ost.noarch

How reproducible:
Always

Steps to Reproduce:
1. Have a VM on a network A
2. openstack network set --mtu 1442 a
3. Live migrate the VM

Actual results:
Fails because MTU of source and target node differs

Expected results:
Succeeds with the new MTU

Additional info:
2021-03-26 15:35:13.615 7 ERROR nova.virt.libvirt.driver [-] [instance: a79d5ec4-d768-4dd0-8c08-bc6f4b07e321] Live Migration failure: unsupported configuration: Target network card MTU 1442 does not match source 1450: libvirt.libvirtError: unsupported configuration: Target network card MTU 1442 does not match source 1450