[vbox-build-flare-vm.py] Improve logs creating subfolder

[Ana06]

Details

Create subfolders within LOGS_DIR with installation timestamp (maybe different from date argument) and VM name in their names instead of deleting previous logs, facilitating debugging of past ans simultaneous runs.

[Ana06]

This is specially important when building a Windows 10 and Windows 11 VM at the same time (or even after each other if you forget to delete the logs). It may be difficult/confusing to know from which build the logs are comming.