storage: improve handling of symlinks in gluster
    
    With this patch, dangling and looping symlinks are silently
    ignored, while links to files and directories are treated the
    same as the underlying file or directory.  This is the same
    behavior as both 'directory' and 'netfs' pools.
    
    * src/storage/storage_backend_gluster.c
    (virStorageBackendGlusterRefreshVol): Treat symlinks similar to
    directory and netfs pools.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>