virtio-devices: Use Default::default() for transitioning to VirtioCommon

In order to simplify the transition to VirtioCommon and to avoid needing
to set empty fields derive Default for VirtioCommon.

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

virtio-devices/src/console.rs

@@ -352,7 +352,7 @@ impl Console {
             Console {
                 common: VirtioCommon {
                     avail_features,
-                    acked_features: 0u64,
+                    ..Default::default()
                 },
                 id,
                 kill_evt: None,