be less conservative with the 64bit pci io window

be less conservative with the 64bit pci io window

Current seabios code will only enable and use the 64bit pci io window in
case it runs out of space in the 32bit pci mmio window below 4G.

This patch will also enable the 64bit pci io window when
  (a) RAM above 4G is present, and
  (b) the physical address space size is known, and
  (c) seabios is running on a 64bit capable processor.

This operates with the assumption that guests which are ok with memory
above 4G most likely can handle mmio above 4G too.

In case the 64bit pci io window is enabled also assign more memory to
prefetchable pci bridge windows and the complete 64bit pci io window.

The total mmio window size is 1/8 of the physical address space.
Minimum bridge windows size is 1/256 of the total mmio window size.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>