storage: zfs: fix double listing of new volumes

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.