Fix VirtualFileQt::seek return value

The libsndfile docu says

sf_vio_seek
  typedef sf_count_t  (*sf_vio_seek)        (sf_count_t offset, int whence, void *user_data) ;

The virtual file context must seek to offset using the seek mode provided by
whence which is one of SEEK_CUR, SEEK_SET, SEEK_END.

The return value must contain the new offset in the file.