vnc: add buffer_free()

Add a buffer_free() helper to free vnc buffers and
remove some duplicated code in vnc_disconnect_finish().

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

vnc.h

@@ -382,6 +382,7 @@ void buffer_reserve(Buffer *buffer, size_t len);
 int buffer_empty(Buffer *buffer);
 uint8_t *buffer_end(Buffer *buffer);
 void buffer_reset(Buffer *buffer);
+void buffer_free(Buffer *buffer);
 void buffer_append(Buffer *buffer, const void *data, size_t len);