virtio_net: return already tracked tx_fifo_errors via virtnet_getstats()

Tx_fifo_errors are tracked in start_xmit_ for virtio_net, but not
reported in the tallies returned by virtnet_stats().  Return them
as the rx "sub-stats" rx_length_errors and rx_frame_errors are.

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>