some disk types do not contain proper partition tables like on time4vps
the type is simfs. On those fdisk fails to access the partition table,
thus being unable to determine the size of the volume.
df does only return the real usable disk space by the user, thus we
lower the 20GB threshold to 18
Fixes#275
Using the bytes output will fix an issue where the disk size is reported
either as terrabyte or also megabyte.
So far we disallowed 1TB disks but allowed 20MB disks.
