Child suspend/resume

Hi all,

The attached patch is completely untested, due to lack of existing
suspendable hardware (no x86 machines).  It does compile cleanly against
head, though. I don't think it should change any behavior, I tried to
keep the essence of the code path the same.

It was suggested that I break up my multipass suspend/resume code into
incremental parts, so this is part one.  It adds a
BUS_SUSPEND_CHILD/BUS_RESUME_CHILD, as well as helper functions,
bus_generic_suspend_child()/bus_generic_resume_child(), and modifies
the PCI driver to use this new facility.

I'd like some feedback, and testing of this, to make sure I didn't
break anything.

Thanks,
Justin