net, virtio_net: replace the magic value

It is more appropriate to use # of queue pairs currently used by
the driver instead of a magic value.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>