iostream: make close() flush

This change is intended to make close() flush the stream before
proceeding with the close itself. This improves the situation
where we have to guarantee that the stream is flushed before
closing it.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>

core/iostream.hh

@@ -173,7 +173,7 @@ public:
     future<> write(net::packet p);
     future<> write(scattered_message<char_type> msg);
     future<> flush();
-    future<> close() { return _fd.close(); }
+    future<> close() { return flush().then([this] { return _fd.close(); }); }
 private:
 };