Spins: list all sizes in GB (power-of-ten)
    
    The only non-power-of-ten size that really makes any sense is
    700 MiB for CDs. DVD size is larger than 4 GiB (which is why
    we use "4.7 GB" for images that target DVD size) and USB sticks
    and SD cards are almost universally sold in power-of-ten sizes.
    There is no logical sense in a size limit of 2 GiB or 4 GiB as
    there are no media that correspond to such a size. relval has
    actually been reading all these "GiB" sizes as GB anyway, and
    all affected images are currently under the power-of-ten size,
    so this should be safe. If any image goes oversize in future,
    the maintainer always has the option of bumping the specified
    size, but we should just make it a rule/convention that it's
    always done in power-of-ten sizes except for CD size.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>