x86: Change how IO port devices are structured in the PC platform.

x86: Change how IO port devices are structured in the PC platform.

Before, for historical reasons, the PCI host device was the default
responder on the IO bus, meaning that when there was any type of
transaction which didn't have a device to go to, it would end up
looking like a PCI config transaction. It's very unlikely that this is
what it actually was, and what would happen would be arbitrary and
probably not helpful.

Also, there was no device in place to respond to accesses in x86's IO
port address space. On a real system, these accesses just return junk
and are otherwise legal. On systems where there would be physical bus
wires they would probably return whatever the last data on the bus was.

This would have been helpful when the platform was first being set up
because it would make it obvious when the OS tried to access a device
that wasn't implemented, but there were a few cases where it would
purposefully fiddle with ports with nothing on them. These had one off
backing devices in the config which would handle the accesses
harmlessly, but if the OS changed and tried to access other ports, the
configs would need to be updated.

Now, the PCI host is just another device on the bus. It claims all of
the PCI config space addresses, so any config access, even ones which
don't go with a device, will go to it, and it can respond with all 1s
like it's supposed to.

In it's place, the default responder is now a bus. On that bus is
a device which responds to the entire IO port address range with 0s.
The default on *that* bus is a device which will mark any accesses
as bad.

With this setup, accesses which don't go to a device, including a
device on the IO port address space, will go to the IO bus's default
port. There, if the access was an IO access, it will go to the device
which replies successfully with all 0s. If not, it's marked as an
error.

The device which backs the entire IO address space doesn't conflict
with the actual IO devices, since the access will only go towards it
if it's otherwise unclaimed, and the devices on the default bus don't
participate in routing on the higher level IO bus.

Change-Id: Ie02ad7165dfad3ee6f4a762e2f01f7f1b8225168
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35515
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>