tests/acceptance/migration: Factor out do_migrate()

We are going to reuse this code when testing different transport
methods, so factor it out first.

Signed-off-by: Oksana Vohchana <ovoshcha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200203111631.18796-2-ovoshcha@redhat.com>
[PMD: Split patch in 2, reworded subject and description]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>