Add 'fall through' comments to case statements without break

These comments are used by static code analysis tools and in code reviews
to avoid false warnings because of missing break statements.

The case statements handled here were reported by coverity.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

json-lexer.c

@@ -301,6 +301,7 @@ static int json_lexer_feed_char(JSONLexer *lexer, char ch, bool flush)
         case JSON_KEYWORD:
         case JSON_STRING:
             lexer->emit(lexer, lexer->token, new_state, lexer->x, lexer->y);
+            /* fall through */
         case JSON_SKIP:
             QDECREF(lexer->token);
             lexer->token = qstring_new();