Move unix specific logic into _unix.go

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/server/imagefs_info.go

@@ -25,6 +25,7 @@ import (
 )
 
 // ImageFsInfo returns information of the filesystem that is used to store images.
+// TODO(windows): Usage for windows is always 0 right now. Support this for windows.
 func (c *criService) ImageFsInfo(ctx context.Context, r *runtime.ImageFsInfoRequest) (*runtime.ImageFsInfoResponse, error) {
 	snapshots := c.snapshotStore.List()
 	timestamp := time.Now().UnixNano()