storage: allow zero capacity with non-backing file to be created
In commit 155ca616e, a change was introduced that no longer allowed defining
volumes via XML with a capacity of '0'. Because we check for info.size_arg
to be non-zero, this use-case fails. This patch allows info.size_arg to be
zero if no backing store is specified.
Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>