iotests: Full mirror to existing non-zero image

The result of a sync=full mirror should always be the equal to the
input.  Therefore, existing images should be treated as potentially
non-zero and thus should be explicitly initialized to be zero
beforehand.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190724171239.8764-12-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>