We are gradually moving away from sector-based interfaces, towards
byte-based.  Update the vdi driver accordingly.  Note that the
TODO is already covered (the block layer guarantees bounds of its
requests), and that we can remove the now-unused s->block_sectors.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>