[libvirt] virStorageVolLookupByPath fails with "libvir: Storage error : invalid storage volume pointer in no storage vol with matching path"

Hello, I have a running KVM virtual guest "demo1" with a defined disk at path /guests/mydisk.img. But when I run conn = virConnectOpenReadOnly(NULL); path = "/guests/mydisk.img"; virStorageVolLookupByPath(conn,path); I got the error " ibvir: Storage error : invalid storage volume pointer in no storage vol with matching path " what am I missing ? Thanks, Francesco