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>

arch/src/riscv64/uefi.rs

@@ -7,7 +7,7 @@ use std::os::fd::AsFd;
 use std::result;
 
 use thiserror::Error;
-use vm_memory::{GuestAddress, GuestMemory};
+use vm_memory::{Bytes, GuestAddress, GuestMemory};
 
 /// Errors thrown while loading UEFI binary
 #[derive(Debug, Error)]