iotests/109: Don't mirror with mismatched size

iotests/109: Don't mirror with mismatched size

This patch makes the raw image the same size as the file in a different
format that is mirrored as raw to it to avoid errors when mirror starts
to enforce that source and target are the same size.

We check only that the first 512 bytes are zeroed (instead of 64k)
because some image formats create image files that are smaller than 64k,
so trying to read 64k would result in I/O errors. Apart from this, 512
is more appropriate anyway because the raw format driver protects
specifically the first 512 bytes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200511135825.219437-2-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>