The last sub-chunk is rounded up to the copy granularity in the target
image, resulting in a larger size than the source.

Add a function to clip the copied sectors to the end.

This undoes the "wrong" changes to tests/qemu-iotests/109.out in
e5b43573. The remaining two offset changes are okay.

[ kwolf: Use DIV_ROUND_UP to calculate nb_chunks now ]
Reported-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>