Fix migrate_instance_*() using DB for floating addresses

This fixes the network api's migrate_instance_start() and
migrate_instance_finish() methods that are still hitting the database
directly for an oddball call to get a list of floating addresses.

Change-Id: Ia310e31d31aaf5c979e41c64af8223202a18e03a
Closes-bug: #1310966
Merge "Fix migrate_instance_*() using DB for floating addresses"