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Qemu should send RARP for vhostuser regardless of whether virtio supports GUEST_ANNOUNCE
Description of problem:
When virtio reports to qemu that GUEST_ANNOUNCE is supported, qemu will not send RARP. The assumption, I think, is that guest kernel will do whatever is needed to announce the guest. The problem with this assumption is that 1) kernel won't send RARPs; 2) for IPv4 and IPv6 it will send GARPs and NAs; 3) for interfaces with no IP addresses, I think it won't send anything at all.
RARPs are useful because they allow to notify regardless of IP configuration. I've [asked](https://issues.redhat.com/browse/RHEL-71919]) RHEL kernel folks to consider issuing RARPs from the guest kernel, but it won't happen overnight, and regardless, it's not a complete solution since we cannot expect all guests running a patched kernel with such feature.
RARP packets are also often expected by underlying network components. For example, OVN controller has a special "activation-strategy=rarp" configuration that makes OVN wait for a RARP from destination chassis on live migration, and only then unblock traffic for the port. Since RARP is not issed by Qemu nor virtio-net, the OVN port is never unblocked (until its configuration is reset by CMS).
I think what should be done from Qemu side is to send RARP for vhostuser ports regardless of whether virtio supports GUEST_ANNOUNCE. I **think** this can be achieved by removing this code:
```
/* If guest supports GUEST_ANNOUNCE do nothing */
if (virtio_has_feature(dev->acked_features, VIRTIO_NET_F_GUEST_ANNOUNCE)) {
return 0;
}
```
Steps to reproduce:
1. Start a VM with vhostuser* port and fresh virtio guest driver.
2. Live migrate it.
3. Observe that RARP is not sent from the migrated port. GARP (or NA for IPv6) is sent instead.
Additional information:
Some external bugs that may be relevant:
- RHEL kernel request to send RARPs from virtio: https://issues.redhat.com/browse/RHEL-71919 (won't fix the issue for older unpatched kernels)
- Request for OVN to handle GARPs and NAs: https://issues.redhat.com/browse/FDP-1042 (won't solve for unaddressed ports!)
- An attempt to work around the issue in OpenStack Neutron OVN env by disabling activation strategy: https://issues.redhat.com/browse/OSPRH-12571 (not a great long term solution)
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