hw/pci-host/q35: Initialize PCMachineState::bus in board code

hw/pci-host/q35: Initialize PCMachineState::bus in board code

The Q35 PCI host currently sets the PC machine's PCI bus attribute
through global state, thereby assuming the machine to be a PC machine.
The Q35 machine code already holds on to Q35's pci bus attribute, so can
easily set its own property while preserving encapsulation.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230630073720.21297-4-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>