slirp: harden restart and DNS handling

slirp: harden restart and DNS handling

Reset transient SLIRP state when the backend is restarted, and make
DHCP DNS advertisement depend on an available resolver.

Keep host loopback resolvers unchanged instead of rewriting them to the
virtual host address. The guest still talks to the SLIRP DNS proxy, but
the proxy must query the resolver configured on the host.

This keeps the emulated network usable after reconfiguration and avoids
advertising a DNS forwarder that cannot answer.