qemu-iotests: Add test for inactive L2 overlap

Extend 060 by a test which creates a corrupted image with an active L2
entry pointing to an inactive L2 table and writes to the corresponding
guest offset.

Also, use overlap-check=all for all tests in 060.

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