qcow2: Use qcow2_signal_corruption() for overlaps

Use the new function in case of a failed overlap check.

This changes output in case of corruption, so adapt iotest 060's
reference output accordingly.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Message-id: 1409926039-29044-4-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
