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/vhost_user/blk.rs

@@ -142,6 +142,7 @@ impl Blk {
             common: VirtioCommon {
                 avail_features,
                 acked_features,
+                ..Default::default()
             },
             id,
             vhost_user_blk,

virtio-devices/src/vhost_user/fs.rs

@@ -358,6 +358,7 @@ impl Fs {
             common: VirtioCommon {
                 avail_features,
                 acked_features,
+                ..Default::default()
             },
             id,
             vu: master,

virtio-devices/src/vhost_user/net.rs

@@ -151,6 +151,7 @@ impl Net {
             common: VirtioCommon {
                 avail_features,
                 acked_features,
+                ..Default::default()
             },
             vhost_user_net,
             kill_evt: None,