Add containerd migration to daemon startup

Add layer migration on startup
Use image size threshold rather than image count
Add daemon integration test
Add test for migrating to containerd snapshotters
Add vfs migration
Add tar export for containerd migration
Add containerd migration test with save and load

Signed-off-by: Derek McGowan <derek@mcg.dev>

daemon/graphdriver/vfs/driver.go

@@ -86,7 +86,16 @@ func (d *Driver) Status() [][2]string {
 
 // GetMetadata is used for implementing the graphdriver.ProtoDriver interface. VFS does not currently have any meta data.
 func (d *Driver) GetMetadata(id string) (map[string]string, error) {
-	return nil, nil
+	dir := d.dir(id)
+	if _, err := os.Stat(dir); err != nil {
+		return nil, err
+	}
+
+	metadata := map[string]string{
+		"SourceDir": dir,
+	}
+
+	return metadata, nil
 }
 
 // Cleanup is used to implement graphdriver.ProtoDriver. There is no cleanup required for this driver.