Allow RPC servers to advertise themselves using MDNS,
via Avahi
* src/rpc/virnetserver.c, src/rpc/virnetserver.h: Allow
registration of MDNS services via avahi
* src/rpc/virnetserverservice.c, src/rpc/virnetserverservice.h: Add
API to fetch the listen port number
* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Add API to
fetch the local port number
* src/rpc/virnetservermdns.c, src/rpc/virnetservermdns.h: Represent
an MDNS advertisement
