PCI BUS stubs

Stub some PCI BUS ioports to return all 1's for read accesses so that
SeaBIOS doesn't find any devices.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Fix PIO reads and writes for non-existing devices

There's plenty devices that we won't support so don't cause the
hypervisor to stop if an ioport doesn't exist. Introduce a
"--ioport-debug" command line that prints out tracing for non-existing
ioports for debugging purposes.

Signed-off-by: Pekka Enberg <penberg@iki.fi>