qemu-iotests: align test requests according to cluster size

qemu-iotests: align test requests according to cluster size

Change the io_test and io_test2 functions to take the cluster size of the image
and the number of test requests to issue. Tests are changed to specify a
cluster size (usually 4k), but expected test results stay the same for now
(apart from qemu-img printing the cluster size now).

Based on a patch written by Christoph Hellwig.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>