Percona: Backup preparation doesn't work ("cannot open ./xtrabackup_logfile")
You have not decompressed the Xtrabackup files if they still have the .qp extension. 

Try using this inside the current folder:

sudo innobackeupex --decompress --parallel=4


where 4 stands for the number of threads you would like to use.

Also, for clean-up afterwards:

sudo find ${DEST_DIR}/ -name "*.qp" -delete