Allow SBI system_reset to fail

ostd/src/arch/riscv/qemu.rs

@@ -14,9 +14,17 @@ pub enum QemuExitCode {
 /// Exit QEMU with the given exit code.
 pub fn exit_qemu(exit_code: QemuExitCode) -> ! {
     log::debug!("exit qemu with exit code {exit_code:?}");
-    match exit_code {
+    let error_code = match exit_code {
         QemuExitCode::Success => sbi_rt::system_reset(sbi_rt::Shutdown, sbi_rt::NoReason),
         QemuExitCode::Failed => sbi_rt::system_reset(sbi_rt::Shutdown, sbi_rt::SystemFailure),
     };
-    unreachable!("qemu does not exit");
+
+    log::error!(
+        "SBI system_reset cannot shut down the underlying machine, error code: {:#?}",
+        error_code
+    );
+
+    loop {
+        riscv::asm::wfi();
+    }
 }