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: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NAlex Williamson <alex.williamson@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180611141716.3813-1-peter.maydell@linaro.org

CODING_STYLE

@@ -124,6 +124,23 @@ We use traditional C-style /* */ comments and avoid // comments.
 Rationale: The // form is valid in C99, so this is purely a matter of
 consistency of style. The checkpatch script will warn you about this.
 
+Multiline comment blocks should have a row of stars on the left,
+and the initial /* and terminating */ both on their own lines:
+    /*
+     * like
+     * this
+     */
+This is the same format required by the Linux kernel coding style.
+
+(Some of the existing comments in the codebase use the GNU Coding
+Standards form which does not have stars on the left, or other
+variations; avoid these when writing new comments, but don't worry
+about converting to the preferred form unless you're editing that
+comment anyway.)
+
+Rationale: Consistency, and ease of visually picking out a multiline
+comment from the surrounding code.
+
 8. trace-events style
 
 8.1 0x prefix