Drop the NetSocketListenState struct and add a listen_fd field
to NetSocketState. When a -netdev socket,listen= instance is created
there will be a NetSocketState with fd=-1 and a valid listen_fd. The
net_socket_accept() handler waits for listen_fd to become readable and
then accepts the connection. When this state transition happens, we no
longer monitor listen_fd for incoming connections...until the client
disconnects again.