During the live migration trunk subports where updated only based
on the "host_id" field. But when rpc message to update subports
is send trunk's port is still bound to the old host and has
"migrating_to" field in binding:profile set to the new host.
Because of that binding:host_id for the subport's port wasn't updated
proberly and port was set to DOWN on the new host.
That could even cause connectivity break if L2population is used in the
cloud.
This patch fixes that by updating subport's binding:host_id field based
on the migrating_to field if that is available and not empty.
Closes-Bug: #1914747
Change-Id: I98e55242d381ada642ca0729e9aefdea7628c945
(cherry picked from commit e05101c8ac)
366fae2d53