engine : Moved SizeConverter to backend utils

SizeConverter is a generic converter which can take the

size in a given i/p unit and convert to a unit of users

choice.It is also capable of rendering size as a string

along with respective units.Moved Size-converter to backend

utils.Moving this is required so that it can be used by

both frontend and backend packages.

Change-Id: I650436e48cdc0fc1c73f86d3424c9f5153e453d0
Signed-off-by: Anmol Babu <anbabu@redhat.com>