Skip user-provided VIP ports in maintenance backfill task

The maintenance task add_device_owner_lb_vip_ports, introduced in change [1] to backfill device_owner and device_id on legacy provider-created VIP ports (LP#2150682), did not account for user-provided ports.

Since change [2] (LP#2160551), users can pass a pre-existing port via –vip-port-id. These ports have empty device_owner/device_id by design, so the maintenance task was incorrectly overwriting them with ovn-lb:vip and lb-, altering ports the user owns.

Port provenance is now determined by the port name prefix (ovn-lb-vip-): only provider-created ports are backfilled. User-provided ports are skipped.