CODING_STYLE: Define our preferred form for multiline comments

CODING_STYLE: Define our preferred form for multiline comments

The codebase has a bit of a mix of different multiline
comment styles. State a preference for the Linux kernel
style:
    /*
     * Star on the left for each line.
     * Leading slash-star and trailing star-slash
     * each go on a line of their own.
     */

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Alex Williamson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 20180611141716[email protected]