Code review comment for lp:~danilo/maas/virsh-storage-units

While we're at it, we could also future-proof ourselves, such as:

CAPACITY_UNITS = {
    "KiB": 2 ** 10,
    "MiB": 2 ** 20,
    "GiB": 2 ** 30,
    "TiB": 2 ** 40,
    "PiB": 2 ** 50,
    "EiB": 2 ** 60,
    "ZiB": 2 ** 70,
    "YiB": 2 ** 80,
}