We are gradually converting to byte-based interfaces, as they are
easier to reason about than sector-based. Convert another internal
function, preserving all existing semantics, and adding one more
assertion that things are still sector-aligned (so that conversions
to sectors in mirror_read_complete don't need to round).
Signed-off-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>