Storage: Introduce shadow vol for refresh while the main vol builds.
    
    Libvirt periodically refreshes all volumes in a storage pool, including
    the volumes being cloned.
    While cloning a storage volume from parent, we drop pool locks. Subsequent
    volume refresh sometimes changes allocation for an ongoing copy, and leads
    to corrupt images.
    Fix: Introduce a shadow volume that isolates the volume object under refresh
    from the base which has a copy ongoing.
    
    Signed-off-by: Prerna Saxena <prerna@linux.vnet.ibm.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>