libvirt.test: Fix hidden failure in test

If domain doesn't have a vnc graphic device, all the
normal tests will be fail. Now before execute vncdisplay
command, test will check if vnc graphic device exists,
if not, test will add a vnc graphic device to domain.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Okey, looks weird. can we save the status before adding a device to guest and restore it at this place? Because we should not deal with the parameter of start_vm, it is out of the duty of a test script. Please assume this scenario, someday we need to do some change about start_vm in env_process. If we do a check of it in this place, we have to update it at the same time. But if we save&restore the status of vm. We can uncouple it.