Env.destroy: Destroy VMs ungracefully

This is an attempt to fix a problem where ./run is trying to log into a
VM that never booted in the first place, making test runs sometimes take
a long time to start.

I am not sure this is really a good idea, but I am assuming that if we
have reached the VM destruction code inside Env.destroy(), that means
all our mechanisms to shut down the VM gracefully have already failed.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>