We are gradually moving away from sector-based interfaces, towards
byte-based.  Make the change for the last few sector-based calls
into the block layer from the replication driver.

Ideally, the replication driver should switch to doing everything
byte-based, but that's a more invasive change that requires a
bit more auditing.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>