qemu-img: Allow creation of zero-sized volumes

In previous version if size is zero it was omitted from the command

Change-Id: Iaf8d493bcf3334bd96d96f5aa02cc564b7b83c9f
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36276
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>