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/device.rs

@@ -183,6 +183,7 @@ pub trait DmaRemapping: Send + Sync {
 }
 
 /// Structure to handle device state common to all devices
+#[derive(Default)]
 pub struct VirtioCommon {
     pub avail_features: u64,
     pub acked_features: u64,