Up to now root has been the parent device for all block devices created via
calling ConnectController(). This does not work well together with the
implementation of bootstd.

Add a dummy parent device for all EFI block devices.

With this change EFI block devices are also accessible via commands like
'cat', 'load', and 'ls'.

    => dm tree
     Class     Seq    Probed  Driver                Name
    -----------------------------------------------------------
     efi           0  [ + ]   EFI block driver      `-- efi
     blk           3  [ + ]   efi_blk                   `-- efi.efiblk#0
     partition     0  [ + ]   blk_partition                 `-- efi.efiblk#0:1

    => ls efiloader 0:1
           13   hello.txt
            7   u-boot.txt
    2 file(s), 0 dir(s)

    => cat efiloader 0:1 hello.txt
    Hello world!

    => efidebug dh
    0000000018df1700 (efi.efiblk#0:1)
      /VenHw(dbca4c98-6cb0-694d-0872-819c650cb7b8)/HD(1,MBR,0xd1535d21...