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>
Indeed, it's strange, for now add_graphic() and del_graphic() are static methods, we can use it with module directly or with an instance, in my test file, it's used with an instance. I think we can leave them here since they are existing static methods.