virtio_net: Set VIRTIO_NET_F_GUEST_CSUM feature

We can handle receiving partial csums, so set the
appropriate feature bit.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>