virtio_net: Make virtio_net support carrier detection

Impact: Make NetworkManager work with virtio_net

For now the semantics are simple: There is always carrier.

This allows a seamless experience with e.g., qemu/kvm
where NetworkManager just configures and sets up
everything automagically.

If/when a generally agreed-upon way to control
carrier on/off in the emulator/hypervisor level
emerges, it will be trivial to extend the driver
to support that too, but for now even this 2-liner
makes user experience that much better.

Signed-off-by: Pantelis Koukousoulas <pktoss@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>