Allow to continue past an interrupted backup run

[jnoss]

Currently if the backup process gets interrupted there might be a partial qcow exported into the QCOW_TEMP_PATH - this will cause subsequent runs to fail as the backup cannot continue if the QCOW_TEMP_PATH is not empty. A fix for this is to have each qcow export go into a new temporary directory inside QCOW_TEMP_PATH, say a folder name generated with uuidgen - then use this full path for the rsnap as well. When done with each image, remove the temporary qcow file and the temporary directory. That way any partial exports or interrupted backups will only orphan this temporary directory but won't cause future runs to fail, as future backups will go into new subdirectories within QCOW_TEMP_PATH.