iotests: Add test for image header overlap

Add a test for an image with an unallocated image header; instead of an
assertion, this should result in the image being marked corrupt.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
