format/render size: allow one to specifiy if base 2 or 10 is desired

format/render size: allow one to specifiy if base 2 or 10 is desired

Storage capacity and usage seems to prefer the base ten (SI unit) use
(makes the capacity a bigger number) while memory (RAM) is normally
preferred to use base 2.

So allow both, mostly to allow consistent displaying of metrics.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>