plat/virtio_net: Notify host of RX bufs on start

plat/virtio_net: Notify host of RX bufs on start

Currently, we don't notify the virtio-net device of available RX buffers
on driver start, meaning it might never send us incoming packets.  Make
sure that we send this notification once the driver has started.

Signed-off-by: Tu Dinh Ngoc <dinhngoc.tu@irit.fr>
Reviewed-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #669