varlink: add new call varlink_server_loop_auto()

varlink: add new call varlink_server_loop_auto()

This is a helper call that runs the specified VarlinkServer object in an
event loop, and exits once no more connections exist.

This is useful for pure varlink servers (i.e. those which only server
varlink requests and do nothing else), to run as long as there's
something to do and exit right after.