The step argument for "add volume <step>" was ignored until now. Fix it.
There is one problem: by defualt, "add volume" should use the value set
with --volstep. This value is 3 by default. Since the default volue for
the step argument is always 1 (and we don't really want to make the
generic code more complicated by introducing custom step sizes), we
simply multiply the step argument with --volstep to keep it compatible.
The --volstep option should probably be just removed in the future.