virtio_net: Update xdp_features with xdp multi-buff

Now virtio-net supports xdp multi-buffer so add it to xdp_features.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/bpf/60c76cd63a0246db785606e8891b925fd5c9bf06.1675763384.git.lorenzo@kernel.org