Skip the default route when an interface has no gateway

[adityasingh2400]

The Interface protocol documents ipv4Gateway and ipv6Gateway as the address for the default route, or nil for no default route, but setupInterface logged the nil-gateway case and then called routeAddDefault anyway. The guest ended up with default dev eth0 scope link, so route lookup treated arbitrary external destinations as directly reachable on the link and started neighbor resolution for them. This also left callers with no way to build a first interface that has no default route.

This returns early when both gateways are nil and leaves every other path untouched, so a v4-only gateway, a v6-only gateway, or a gateway outside the subnet all still install the same routes as before.

The new tests drive setupInterface through a recording VirtualMachineAgent double, which covers the routing decisions without booting a sandbox. noDefaultRouteWhenBothGatewaysAreNil fails on main and passes here, and defaultRouteInstalledForIPv6OnlyGateway pins the v6-only behavior that the existing integration test testIPv6OnlyDefaultRoute exercises against a real VM.

Fixes #825

[adityasingh2400]

One thing worth flagging for review: vminitd's ipRouteAddDefault has a branch that deliberately installs a gatewayless v4 default route when the request carries no v4 gateway and no v6 gateway, commented as preserving pre-IPv6 behavior. I left that guest-side branch alone on purpose, so an older host talking to a newer vminitd keeps the exact behavior it has today and the wire contract does not change. The fix is host-side only, which is also where the Interface documentation lives.

Happy to move the check into the guest instead, or to gate it behind an explicit opt-in on Interface, if you would rather the compatibility path be retired at the same time.