PCI express support has been made available for quite sometime now with patches from Isaku Yamahata for the Q35 chipset for Qemu 0.12 and then for Qemu 0.14. However, Qemu 0.15 has been released but support for the Q35 chipset is still not officially included.
At what point will Qemu merge the Q35 chipset code for PCIexpress? AK
