The attached patch exposes the exception thrown by xen.lowlevel.xc as 
the type xen.lowlevel.xc.Error which is an exception that inherits from 
RuntimeError.

I only had a few minutes this morning so I didn't get to xen.lowlevel.xs 
but hopefully someone else can use this as a guide on what needs to be 
done.  If noone else gets to it, I'll be able to submit another one in 
about a week.

I've only done very basic testing but I don't expect that this should 
break anything...

Regards,

Anthony Liguori