unbreak ppc/prep

Changes:
  * added isa bus, hooked up to the system bus. Not sure this is correct,
    but 'info pci' lists lists no pci-isa bridge in the machine ...).
  * switches the default cpu to one which actually works.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>