vmm: fix riscv64 legacy serial MMIO setup

Signed-off-by: Zewei Yang <yangzewei@loongson.cn>

Author: yzewei

src/vmm/src/builder.rs

@@ -943,7 +943,8 @@ pub fn build_microvm(
             &vm,
             &mut mmio_device_manager,
             &mut kernel_cmdline,
-            serial_device,
+            intc.clone(),
+            serial_devices,
         )?;
     }
 

src/vmm/src/device_manager/kvm/mmio.rs

@@ -199,7 +199,10 @@ impl MMIODeviceManager {
         vm.register_irqfd(serial.lock().unwrap().interrupt_evt(), self.irq)
             .map_err(Error::RegisterIrqFd)?;
 
+        #[cfg(target_arch = "aarch64")]
         serial.lock().unwrap().set_intc(intc);
+        #[cfg(target_arch = "riscv64")]
+        let _ = intc;
 
         self.bus
             .insert(serial, self.mmio_base, MMIO_LEN)