Deprecate default IP address + mask support for virtio devices

[mguentner]

It should be possible to create empty tap devices without an IP and mask.

cloud-hypervisor automatically sets a default static IP address (192.168.249.1) if none is provided.
Alternatively, a dummy IP can be used which is later removed again by some other means.

In any case, this could lead to:

• multiple devices without explicit IP configurations would end up with the same default IP
• unnecessary inclusion of such IPs in firewall rules
• the IP address could clash with host networking and routing

#7076 is a proposal to remove these constraints.

The API in terms of parameters would not change as the default address is silently assigned at the moment, however users that expect the current behavior would be surprised.

A warning should therefore be added in either the parsing function (pub fn parse(net: &str)) or better, wherever NetConfig is first constructed (parse in ch-remote, after VmConfig::parse in start_vmm) / de-serialized (serde_json::from_slice in http_endpoint).

CC @rbradford

[likebreath]

IIUC, the proposal here is to remove the hard-coded default ip and mac address we have for virtio-net devices. I think that makes sense. If that makes sense, I'd rename the issue to "Deprecate default ip and mac address mask support for virtio devices".

Also, as @rbradford suggested, since this is about deprecating a feature, we need to warn users ahead and finally remove it in two release.

@mguentner Would you please kick-off this work with a PR to add such warning? Thank you.

[mguentner]

Actually it's just about deprecating the static, default IP address + mask.
Generating a default MAC address when none is provided makes sense.