arch, vmm: Fix riscv64 build gaps

Several riscv64 paths are compiled by the KVM build but missed
imports or cfg coverage needed by the current code.

Import the vm-memory Bytes trait for the RISC-V UEFI loader, keep
Instant available for migration timing code, and enable the UEFI flash
error path for riscv64.

Assisted-by: OpenAI-Codex:GPT-5

Signed-off-by: wangyf0611 <wangyufeng@iscas.ac.cn>

vmm/src/lib.rs

@@ -12,9 +12,7 @@ use std::panic::AssertUnwindSafe;
 use std::path::PathBuf;
 use std::sync::mpsc::{Receiver, RecvError, SendError, Sender};
 use std::sync::{Arc, Mutex};
-use std::time::Duration;
-#[cfg(not(target_arch = "riscv64"))]
-use std::time::Instant;
+use std::time::{Duration, Instant};
 use std::{io, result, thread};
 
 use anyhow::{Context, anyhow};