libxl: Document ~/serial/ correctly

libxl: Document ~/serial/ correctly

xenstore-paths.markdown talked about ~/device/serial/, but that's not
used.

(It is very wrong for this value, which contains a driver domain
filesystem path, to be in the guest's area of xenstore.  However, it
is only ever created by libxl and ready by xenconsoled.  When it is
created, it inherits the read-only permissions of /local/domain/DOMID.
So there is no security bug.)

This is a followup to XSA-175.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>