virtio-devices: net: add guest-announce plumbing

Advertise `VIRTIO_NET_F_GUEST_ANNOUNCE` on virtio-net devices, surface
`VIRTIO_NET_S_ANNOUNCE` through config status, and handle
`VIRTIO_NET_CTRL_ANNOUNCE_ACK` on the control queue.

This adds the guest-visible state needed for post-migration or
post-restore announce requests; the VMM side triggering is added in
follow-up commits.

The motivation is to reduce post-migration and post-restore
connectivity gap. After a live migration or after restoring, it can
take the guest several seconds to be reachable again over the network.
With these announcements, the network path should be refreshed within a
few milliseconds.

On-behalf-of: SAP sebastian.eydam@sap.com
Signed-off-by: Sebastian Eydam <sebastian.eydam@cyberus-technology.de>

net_util/src/lib.rs

@@ -101,7 +101,7 @@ fn create_unix_socket() -> Result<net::UdpSocket> {
     Ok(unsafe { net::UdpSocket::from_raw_fd(sock) })
 }
 
-fn vnet_hdr_len() -> usize {
+pub fn vnet_hdr_len() -> usize {
     size_of::<virtio_net_hdr_v1>()
 }