virtio: virtqueue_get_avail_bytes: fix desc_pa when loop over the indirect descriptor table

virtqueue_get_avail_bytes: when found a indirect desc, we need loop over it.
           /* loop over the indirect descriptor table */
           indirect = 1;
           max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
           num_bufs = i = 0;
           desc_pa = vring_desc_addr(desc_pa, i);
But, It init i to 0, then use i to update desc_pa. so we will always get:
desc_pa = vring_desc_addr(desc_pa, 0);
the last two line should swap.

Cc: qemu-stable@nongnu.org
Signed-off-by: NYin Yin <yin.yin@cs2c.com.cn>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -377,8 +377,8 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
             /* loop over the indirect descriptor table */
             indirect = 1;
             max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
-            num_bufs = i = 0;
             desc_pa = vring_desc_addr(desc_pa, i);
+            num_bufs = i = 0;
         }
 
         do {