iotests: Add more tests for qcow2 corruption

Add tests for unaligned L1/L2/reftable entries and non-fatal corruption
reports.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1409926039-29044-6-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
