fix backup duration per VM

[tatref]

time_start is defined once at the beginning of the script, so the duration displayed is not per VM. Also the time is increasing:

2022-01-12 22:32:38,860: Duration: 2:37 minutes
2022-01-12 22:35:58,685: Duration: 5:57 minutes
2022-01-12 22:38:46,261: Duration: 8:45 minutes
2022-01-12 22:41:08,629: Duration: 11:7 minutes
2022-01-12 22:44:16,983: Duration: 14:15 minutes
2022-01-12 22:47:10,029: Duration: 17:9 minutes

I simply moved the initialization of the timer inside the vm loop. Maybe we could also add a global timer?

[wefixit-AT]

please check the conflicts