backup: Switch BackupBlockJob to byte-based

We are gradually converting to byte-based interfaces, as they are
easier to reason about than sector-based.  Continue by converting an
internal structure (no semantic change), and all references to
tracking progress.  Drop a redundant local variable bytes_per_cluster.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>