hypervisor: arch: Integrate riscv64 to arch module

Integrate `aia` module into `riscv64` module, and enable `riscv64`
module if target architecture is RISC-V 64-bit.

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

hypervisor/src/arch/mod.rs

@@ -21,3 +21,6 @@ pub mod x86;
 
 #[cfg(target_arch = "aarch64")]
 pub mod aarch64;
+
+#[cfg(target_arch = "riscv64")]
+pub mod riscv64;

hypervisor/src/arch/riscv64/mod.rs

@@ -0,0 +1,5 @@
+// Copyright © 2024 Institute of Software, CAS. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+pub mod aia;