lookup hard interrupt events by number.  Ignore the irq# for soft intrs.
 - Add support to cpuset for binding hardware interrupts.  This has the
   side effect of binding any ithread associated with the hard interrupt.
   As per restrictions imposed by MD code we can only bind interrupts to
   a single cpu presently.  Interrupts can be 'unbound' by binding them
   to all cpus.

Reviewed by:	jhb
Sponsored by:	Nokia