Match backup target directory to virtual machine

[abbbi]

Currently following might possible if virtual machine has the same amount of checkpoints:

virtnbdbackup -l full -d vm1 -o /tmp/backup
virtnbdbackup -l inc -d vm2 -o /tmp/backup

its not checked wether if the already existing data-set in the target directory matches the virtual machine
that is backed up.

[abbbi]

Wont work bcs cbt file is named after virtual machine and backup wont find any checkpoints:

./virtnbdbackup -l inc -d vm4 -o /tmp/vm2b/
2021-05-03 13:45:13 INFO common - printVersion: Version: 0.19 Arguments: ./virtnbdbackup -l inc -d vm4 -o /tmp/vm2b/
2021-05-03 13:45:13 INFO virtnbdbackup - main: Domain has 1 disks attached which support changed block tracking.
2021-05-03 13:45:13 INFO virtnbdbackup - main: Looking for checkpoints
2021-05-03 13:45:13 ERROR virtnbdbackup - main: No prior checkpoints found, execute full backup first

so accidentally writing files from one vm to another vm backup directory wont happen.