if no lapic available, use 8253pit for clock
now xv6 runs both on qemu (uniprocessor) and bochs (uniprocessor and MP)

don't use the same name for two different pieces of code

ioapic

Undo change from this morning that wasn't intended to get in.

disk interrupts (assuming bochs has a bug)

and plan 9 code, at least boots and gets into C code.