conf: use common struct in storage volumes
    
    A fairly smooth transition.  And now that domain disks and
    storage volumes share a common struct, it opens the doors for
    a future patch to expose more details in the XML for both
    objects.
    
    * src/conf/storage_conf.h (_virStorageVolTarget): Delete.
    (_virStorageVolDef): Use common type.
    * src/conf/storage_conf.c (virStorageVolDefFree)
    (virStorageVolTargetDefFormat): Update clients.
    * src/storage/storage_backend.h: Likewise.
    * src/storage/storage_backend.c
    (virStorageBackendDetectBlockVolFormatFD)
    (virStorageBackendUpdateVolTargetInfo)
    (virStorageBackendUpdateVolTargetInfoFD): Likewise.
    * src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
    Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>