block/export: fix vhost-user-blk export sector number calculation

The driver is supposed to honor the blk_size field but the protocol
still uses 512-byte sector numbers. It is incorrect to multiply
req->sector_num by blk_size.

VIRTIO 1.1 5.2.5 Device Initialization says:

  blk_size can be read to determine the optimal sector size for the
  driver to use. This does not affect the units used in the protocol
  (always 512 bytes), but awareness of the correct value can affect
  performance.

Fixes: 3578389 ("block/export: vhost-user block device backend server")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210223144653.811468-10-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/vhost-user-blk-server.c

@@ -144,7 +144,7 @@ static void coroutine_fn vu_blk_virtio_process_req(void *opaque)
             break;
         }
 
-        int64_t offset = req->sector_num * vexp->blk_size;
+        int64_t offset = req->sector_num << VIRTIO_BLK_SECTOR_BITS;
         QEMUIOVector qiov;
         if (is_write) {
             qemu_iovec_init_external(&qiov, out_iov, out_num);