avocado.plugins.vm: group command line options.

Group command line options regarding virtualization together.

virtualization arguments:
  --vm                  Run tests on Virtual Machine
  --vm-hypervisor-uri VM_HYPERVISOR_URI
                        Specify hypervisor URI driver connection
  --vm-domain VM_DOMAIN
                        Specify domain name (Virtual Machine name)
  --vm-hostname VM_HOSTNAME
                        Specify VM hostname to login
  --vm-username VM_USERNAME
                        Specify the username to login on VM
  --vm-password VM_PASSWORD
                        Specify the password to login on VM
  --vm-cleanup          Restore VM to a previous state, before running the
                        tests

Signed-off-by: Ruda Moura <rmoura@redhat.com>