s390x/a-b-bios: zero the first byte of each page on start

s390x/a-b-bios: zero the first byte of each page on start

Same as with the x86 verison of this test, we relied on the contents of
all pages in RAM to be the same across the entire test range, which is
very fragile. Zero the first byte of each page before running the
increment loop to fix this.

Fixes: 5571dc824b ("tests/migration: Enable the migration test on s390x, too")
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20230919102346.2117963-4-d-tatianin@yandex-team.ru>