[VirtualBox/f26] (5 commits) ...Small fixes

Summary of changes: 84eba00... Don't build guest-additions for F28+ (now it is available o (*) 3d2e8bf... Fix build (*) 5eca1fc... Update VBox to 5.2.12 (*) ed41096... Update VBox to 5.2.12 Previous bug shows that we don't need (*) 00a4be0... Small fixes (*) (*) This commit already existed in another branch; no separate mail sent