iotests: Improve image-clear tests on non-aligned image

iotests: Improve image-clear tests on non-aligned image

Tweak 097 and 176 to operate on an image that is not cluster-aligned,
to give further coverage of clearing out an entire image, including
the recent fix to eliminate the difference between fast path (97) and
slow (176) for qcow2.  Also tested on qcow (97 only, since qcow lacks
snapshots).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170331185356.2479-4-eblake@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>