Add ability to clone complete volumes to a new path

There's a clone() function for volumes already, but that seems to only clone the Fog metadata to a new blank volume. I need to clone the actual disk, so this patch adds clone_volume to accomplish that

Also, my libvirt hypervisor seems to report capacities/allocations as "10" instead of "10G", meaing the unit detection crashes - I've added a separate commit for that,