LoongArch: KVM: Add hypercall instruction emulation

On LoongArch system, there is a hypercall instruction special for
virtualization. When system executes this instruction on host side,
there is an illegal instruction exception reported, however it will
trap into host when it is executed in VM mode.

When hypercall is emulated, A0 register is set with value
KVM_HCALL_INVALID_CODE, rather than inject EXCCODE_INE invalid
instruction exception. So VM can continue to executing the next code.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>