consomme: ipv6 address discoverability updates

[Brian-Perkins]

Consomme updates to expand ipv6 address discovery from the guest.

Adapter can be marked to not advertise routable address availability if traffic on that adapter is only going to be used for local/loopback traffic.

Infer Ipv6 link local address from routable address if the link local address is not yet known and the routable address conforms to a EUI-64 address derived from the guest MAC.

Discover the link local address from sent packets, similar to the routable address.

Additional tracing.

[Copilot]

Pull request overview

This PR updates the net_consomme device’s IPv6 address discovery and NDP behavior to improve guest IPv6 address “learnability” and to optionally suppress advertising a routable SLAAC prefix on adapters intended for local/loopback-only traffic.

Changes:

• Add advertise_routable_ipv6 to control whether Router Advertisements include Prefix Information for SLAAC-based routable address creation.
• Improve guest IPv6 address discovery by learning the guest’s link-local address from traffic and inferring link-local from a learned routable EUI-64-based SLAAC address.
• Add more detailed ICMPv6/NDP tracing and a dedicated UnsupportedIcmpv6 drop reason for non-NDP ICMPv6.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
vm/devices/net/net_consomme/src/lib.rs	Treat UnsupportedIcmpv6 drops as “unknown” TX results in queue stats.
vm/devices/net/net_consomme/consomme/src/ndp.rs	Add NDP receive tracing and gate RA prefix advertisement on advertise_routable_ipv6.
vm/devices/net/net_consomme/consomme/src/lib.rs	Introduce advertise_routable_ipv6, link-local inference from routable EUI-64, link-local learning from traffic, and UnsupportedIcmpv6.