hw/pci-host/gpex: Implement PCI INTx routing

Now we are able to retrieve the gsi from the INTx pin, let's
enable intx_to_irq routing. From that point on, irqfd becomes
usable along with INTx when assigning a PCIe device.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Tushar Jagad <tushar.jagad@linaro.org>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Feng Kan <fkan@apm.com>
Message-id: 1505296004-6798-4-git-send-email-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>