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>
Yes, you are right! I will fix it, thanks a lot!