Add variant blivet 'part_fs' needle, rename the other
    
    When running universal tests with a non-Server image, which is
    not common but *does* happen (e.g. manually built test image),
    the pre-selected filesystem in blivet-gui will be ext4 not xfs
    (just as with custom partitioning). So we need xfs and ext4
    variants of this needle. Renamed the XFS one for consistency.