runtime: Define availableGuestProtection for riscv64

`GuestProtection` feature is not made available yet, return
`noneProtection` for now.

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

src/runtime/virtcontainers/hypervisor_linux_riscv64.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2024 Institute of Software, CAS.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package virtcontainers
+
+// Guest protection is not available on RISC-V.
+func availableGuestProtection() (guestProtection, error) {
+	return noneProtection, nil
+}