tests/qtest/migration-test: Fix and enable test_ignore_shared

tests/qtest/migration-test: Fix and enable test_ignore_shared

This test is already starting to bitrot, so first remove it from ifdef
and fix compile issues. ppc64 transfers about 2MB, so bump the size
threshold too.

It was said to be broken on aarch64 but it may have been due to the
limited shm size under Gitlab CI.

Cc: Yury Kotov <yury-kotov@yandex-team.ru>
Cc: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
[put it under flaky tests, we still don't have a solution for the CI]
Signed-off-by: Fabiano Rosas <farosas@suse.de>