OvmfPkg/VirtioFsDxe: split canon. path into last parent + last component
    
    Given a canonical pathname (as defined by VirtioFsAppendPath()), different
    from "/", introduce a helper function for:
    
    - looking up the NodeId of the most specific parent directory, and
    
    - exposing the last component stand-alone (which is therefore a direct
      child of said parent directory).
    
    This splitting operation will be necessary in multiple subsequent patches.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>