allow --size to be an float like 1.5G

In my ignorance i tried to create an image with something like ./bin/debvm-create -z 1.2G -r bookworm -k ~/.ssh/id_rsa.pub -- --hook-dir=./useraddhook --include sudo,vim,zstd which resulted in an rootfs.ext4 with the size of 536M which i think is just the used space.

Maybe the numfmt call could be improved tho.