Introduce a basic vnc test for qemu

This vnc test is used to verify:
 * Basic vnc connection.
 * qemu monitor commands of setting vnc password.
 * qemu monitor commands of setting vnc password expiration.

Note: This test can't work with old qemu version which doesn't provide `set_password` and `expire_password` monitor cmd. Thus I disable this test on RHEL5 host.