MIPS: KVM: Add kvm guest support for Loongson-3

Loongson-3 KVM guest is based on virtio, it use liointc as its interrupt
controller and use GPEX as the pci controller.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>