hypervisor: kvm: Introduce riscv64 register g/set

Implement macros to calculate register ID on riscv64, definition of
RISC-V `VcpuKvmState`.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

hypervisor/src/kvm/mod.rs

@@ -65,6 +65,9 @@ use crate::{
 // aarch64 dependencies
 #[cfg(target_arch = "aarch64")]
 pub mod aarch64;
+// riscv64 dependencies
+#[cfg(target_arch = "riscv64")]
+pub mod riscv64;
 #[cfg(target_arch = "aarch64")]
 use std::mem;