Move get_fssize() to arguments.c as it is used by lxc_clone. and lxc_create.c,
and rename it to get_fssize_argument()

Signed-off-by: Norberto Bensa <nbensa@gmail.com>
Fix SI (International System of Units/Le Système international d'unités) units
in LVM functions.

1GB = 1000000000 bytes, SI standard, 10^9 bytes.
1GiB = 1073741824 bytes, a.k.a. "binary units", 2^30 bytes.

As of 2013-07-13, LVM defaults to binary units. Fix the code to reflect this
fact.

Signed-off-by: Norberto Bensa <nbensa@gmail.com>