Block layer may recursively check block_status in file child of qcow2,
if qcow2 driver returned DATA. There are several test cases to check
influence of lseek on block_status performance. To see real difference
run on tmpfs.

Tests originally created by Kevin, I just refactored and put them
together into one executable file with simple output.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>