At the level of simulation provided by Simics, there is very little difference between a PCI bus and a PCI Express switch. The major change is the extension of the configuration registers to 4096 bytes per device instead of 256. This is taken into account if you define a PCI Express system using the pcie-switch class instead of the pci-bus class. The PCIe switch will check if the device claims to be PCIe compatible (whether it implements the pci_express interface or not) before allowing it to be connected. Other PCIe features depend on the device used as root complex for the system.