storage: Rename encryption info variable for clarity
    
    Change from @enc to @encinfo leaving @enc for the vol->target.encryption
    in the storageBackendCreateQemuImgSetOptions code path.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>
    ACKed-by: Michal Privoznik <mprivozn@redhat.com>