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>

virtio-devices/src/lib.rs

@@ -15,6 +15,7 @@ use std::{fmt, io, result};
 
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
+use vmm_sys_util::errno::Error as ErrnoError;
 
 #[macro_use]
 mod device;
@@ -122,6 +123,8 @@ pub enum ActivateError {
     CreateRateLimiter(#[source] io::Error),
     #[error("Failed to activate the vDPA device")]
     ActivateVdpa(#[source] vdpa::Error),
+    #[error("Failed to create TimerFd")]
+    CreateTimerFd(#[source] ErrnoError),
 }
 
 pub type ActivateResult = result::Result<(), ActivateError>;