Add graphics.vnc feature test.

Test of libvirt VNC related features.

1) Block specified ports if required;
2) Setup VNC TLS certification if required;
3) Setup graphics tag in VM;
4) Try to start VM;
5) Parse and check result with expected.
6) Clean up environment.

This PR depends on https://github.com/autotest/virt-test/pull/1836

Signed-off-by: Hao Liu <hliu@redhat.com>