vm-virtio: Add shutdown method to VirtioDevice trait

This allows the VMM to explicitly shutdown devices as part of the VM
shutdown ahead of what Drop::drop() would do.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-virtio/src/device.rs

@@ -101,6 +101,12 @@ pub trait VirtioDevice: Send {
     fn iommu_translate(&self, addr: u64) -> u64 {
         addr
     }
+
+    /// Some devices may need to do some explicit shutdown work. This method
+    /// may be implemented to do this. The VMM should call shutdown() on
+    /// every device as part of shutting down the VM. Acting on the device
+    /// after a shutdown() can lead to unpredictable results.
+    fn shutdown(&mut self) {}
 }
 
 /// Trait providing address translation the same way a physical DMA remapping