test-driver.py: defaulting keepVmState in Machine init
test-driver.py: defaulting keepVmState in Machine init

ecb73fd5557d6d438aa7c155e5b1aad89373c6ae introduced a new keepVmState
CLI flag for test-driver.py. This CLI flags gets forwarded to the
Machine class through create_machine.

It created a regression for the boot tests where __main__ end up not
being evaluated. See
https://github.com/NixOS/nixpkgs/pull/97346#issuecomment-690951837 for
bug report.

Defaulting keepVmState to false when __main__ ends up not being
evaluated.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs