Allow for reconnection after the client disconnects

It'd be nice to give the user the possibility to choose if virtiofsd should retry to listen on the same socket after the connection from the client side has been closed. In practice, there are a few use case where this can be useful, like VM reboot or a potential VM migration (on the same host). When these use cases are triggered, the VM might close the connection with the vhost-user backend, leading to the termination of the virtiofsd process. Adding a CLI option --keep-listening to virtiofsd would give the user a way to choose if the virtiofsd process should terminate or keep listening after the client (the VMM) disconnected.