[XeTeX] fontspec and fonts without certain styles

[maxwell]

For many of the fonts we use, there is no italic version (and in some 
cases no bold).  This is natural for Arabic and Thaana scripts, since 
there is no tradition of italicizing them.  (Nasta'liq is quite a 
different sort of style...)

When I include a font loading command for one of these fonts, like--
     \setmainfont[Script=Arabic]{Scheherazade}
or
     \newfontfamily\naskhfont[Script=Arabic]{Scheherazade}
--fontspec outputs something that could be taken as a warning, but is 
probably more like information:
     Could not resolve font "Scheherazade/I" (it probably doesn't exist).

Is there a way to tell fontspec not to bother looking for an italicized 
(or bold) version of particular fonts, and thereby prevent its 
outputting this message?  Adding the parameter
     ItalicFeatures={}
(where the {} would normally enclose the path to a font file) does not 
turn off the message.

I looked at the code for the \@@_find_autofonts macro in 
fontspec-internal.dtx, but it did not enlighten me.

Maybe I should just not worry...

MWE follows my sig line.

     Mike Maxwell
     University of Maryland

-----------MWE---------
\documentclass{article}
\usepackage{fontspec}
\setmainfont[Script=Arabic]{Scheherazade}

\begin{document}
سلام
\end{document}