runtime: Introduce riscv64 to govmm pkg

Define `vmm` for riscv64, set `MaxVCPUs` to 512 as QEMU RISC-V virt
Generic Virtual Platform [1] define.

[1] https://www.qemu.org/docs/master/system/riscv/virt.html

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

src/runtime/pkg/govmm/vmm_riscv64.go

@@ -0,0 +1,12 @@
+//
+// Copyright (c) 2024 Institute of Software, CAS.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package govmm
+
+// MaxVCPUs returns the maximum number of vCPUs supported
+func MaxVCPUs() uint32 {
+	return uint32(512)
+}