pci: Allocate PCIe host bridge PCI ID

We are going to introduce a PCIe host controller that doesn't exist that
way in real hardware, but still needs to expose some PCIe root device which
has PCI IDs.

Allocate a PCI ID in the Red Hat space that we use for other devices of this
kind.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
