This is a follow-up of [1]. Before this patch, any virtual logical
switch port that was updated and the "device_owner" was not and empty
string, had its type set to '' (empty string).
This maintenance task, that is executed only once, lists all logical
switch ports, checks the presence or not of virtual parents and
sets the type to "virtual" if needed.
Related-Bug: #1973276
[1]https://review.opendev.org/c/openstack/neutron/+/841711
Conflicts:
neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/maintenance.py
neutron/tests/unit/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_maintenance.py
Change-Id: I6cf1167d556f0c2c2aa2013f05c809648020b377
(cherry picked from commit 3c93da7bdf)