storage: zfs: fix double listing of new volumes
    
    Currently, after calling commands to create a new volumes,
    virStorageBackendZFSCreateVol calls virStorageBackendZFSFindVols that
    calls virStorageBackendZFSParseVol.
    
    virStorageBackendZFSParseVol checks if a volume already exists by
    trying to get it using virStorageVolDefFindByName.
    
    For a just created volume it returns NULL, so volume is reported as
    new and appended to pool->volumes. This causes a volume to be listed
    twice as storageVolCreateXML appends this new volume to the list as
    well.
    
    Fix that by passing a new volume definition to
    virStorageBackendZFSParseVol so it could determine if it needs to add
    this volume to the list.