lxc_clone.c: Allow size subfixes for -L parameter

lxc-clone ignores size subfixes (K, M, G) when using -L parameter. The
following is a quick patch to allow, for example, lxc-clone -L 10G.

Signed-off-by: Norberto Bensa <nbensa@gmail.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>