Storage: Introduce shadow vol for refresh while the main vol builds.

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>