Add ucall test support for LoongArch, ucall method on LoongArch uses
undefined mmio area. It will cause vCPU exiting to hypervisor so that
hypervisor can communicate with vCPU.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>