Now that there are no remaining clients, we can drop the
sector-based blk_read(), blk_write(), blk_aio_readv(), and
blk_aio_writev().  Sadly, there are still remaining
sector-based interfaces, such as blk_*discard(), or
blk_write_compressed(); those will have to wait for another
day.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>