block: Switch bdrv_make_zero() to byte-based

block: Switch bdrv_make_zero() to byte-based

We are gradually converting to byte-based interfaces, as they are
easier to reason about than sector-based.  Change the internal
loop iteration of zeroing a device to track by bytes instead of
sectors (although we are still guaranteed that we iterate by steps
that are sector-aligned).

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: John Snow <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>