char: free the tcp connection data when closing

Make sure the connection data got freed when closing the chardev, to
avoid leaks. Introduce tcp_chr_free_connection() to clean all connection
related data, and move some tcp_chr_close() clean-ups there.

(while at it, set write_msgfds_num to 0 when clearing array in
tcp_set_msgfds())

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
