tweak 'full' disk image to avoid #2214342
    
    With the new 2G EFI system partition size, the delete_partial
    test that uses this disk image is failing. We've reported the
    bug, but we don't want the test to fail the same way every day,
    we want to catch any other bugs that show up. Tweaking the disk
    image so it has a 6G and a 4G partition (instead of two 5G
    partitions) and the test uses the 6G partition seems to work
    around the problem, so let's do that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>