virttest.env_process: Remove restart_vm option

The restart_vm option, that will restart a vm at the beginning
of postprocessing, is unnecessary, given that you could
simply shut down all the VMs at the end of each test,
forcing new vms to be started.

So get rid of this option, and replace --restart-vm
in ./run to use kill_vm = yes instead.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>