block: Use bdrv_nb_sectors() in bdrv_make_zero() Instead of bdrv_getlength(). Variable target_size is initially in bytes, then changes meaning to sectors. Ugh. Replace by target_sectors. Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Benoit Canet <benoit@irqsave.net> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>