virtio: use unsigned int for counting bytes in vq

The virtqueue_avail_bytes() function counts bytes in an int.  Use an
unsigned int instead.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio.c

@@ -338,7 +338,7 @@ static unsigned virtqueue_next_desc(target_phys_addr_t desc_pa,
 int virtqueue_avail_bytes(VirtQueue *vq, int in_bytes, int out_bytes)
 {
     unsigned int idx;
-    int total_bufs, in_total, out_total;
+    unsigned int total_bufs, in_total, out_total;
 
     idx = vq->last_avail_idx;