qcow2: Tighten cluster_offset alignment assertions

qcow2: Tighten cluster_offset alignment assertions

qcow2_alloc_cluster_offset() and qcow2_get_cluster_offset() always
return offsets that are cluster-aligned so don't just check that they
are sector-aligned.

The check in qcow2_co_preadv_task() is also replaced by an assertion
for the same reason.

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 558ba339965f858bede4c73ce3f50f0c0493597d.1579374329[email protected]
Signed-off-by: Max Reitz <[email protected]>