tests/acceptance/migration: Test UNIX transport when migrating

Along with VM migration via TCP, we can use migration through
the UNIX transport protocol

Signed-off-by: Oksana Vohchana <ovoshcha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200203111631.18796-3-ovoshcha@redhat.com>
[PMD: Split patch in 2, reworded subject and description]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>