qemu-img: avoid unaligned read requests during convert

in case of large continous areas that share the same allocation status
it happens that the value of s->sector_next_status is unaligned to the
cluster size or even request alignment of the source. Avoid this by
stripping down the s->sector_next_status position to cluster boundaries.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <20200901125129.6398-1-pl@kamp.de>
[mreitz: Disable vhdx for 251]
Signed-off-by: Max Reitz <mreitz@redhat.com>