tests/acceptance/migration.py: Wait for both sides

tests/acceptance/migration.py: Wait for both sides

When the source finishes migration the destination will still be
receiving the data sent by the source, so it might not have quite
finished yet, so won't quite have reached 'completed'.
This lead to occasional asserts in the next few checks.

After the source has finished, check the destination as well.
(We can't just switch to checking the destination, because it doesn't
give a status until it has started receiving the migration).

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200528112404.121972-1-dgilbert@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>