virtio_net: rename driver struct to please modpost

Commit

	3d1285b (move virtnet_remove to .devexit.text)

introduced the first reference to __devexit in struct virtio_driver
virtio_net which upset modpost ("Section mismatch in reference from the
variable virtio_net to the function .devexit.text:virtnet_remove()").

Fix this by renaming virtio_net to virtio_net_driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Blame-taken-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>