OVN trunk subport bouncing between compute while live-migrating

[Arnau Verdaguer]

While live-migrating a trunk VM, during the migration the subport will bounce between
the old compute and the new compute. At the end of the migration the subport will be
bound to the correct compute and it will work fine.

During the live-migration of a simple VM with a trunk port and a trunk subport you can
observe multiple PortBindingChassisEvent for a single subport:
```
[stack@undercloud-0 tmp]$ cat 1/server.log.1 | grep 'PortBindingChassisEvent' | grep 7705625c-1659-44f8-a045-346b0512a82a | wc -l
9
```

This issue is caused since OVN doesn't set the LSP_OPTIONS_REQUESTED_CHASSIS_KEY on the subport's options unlike it's done
on the parent port