VNC support should be available by default. So as you said there may be a problem in firewall or qemu/libvirt configuration.
Regarding qemu by default it only accepts local vnc connections. To change that edit
vnc_listen = "0.0.0.0"
This will enable connections from all hosts, which depending what you are trying to do can be very dangerous. In that file you can find more info on how to secure things up.
Also, if you use libvirt I think it also only accepts local connections by default. To enable remote access edit
listen_tcp = 1
The same security concerns regarding qemu apply here. See how to secure the connection if needed here.
As a last resort check if the ports used by vnc and libvirt are open. In the case you are using ufw:
sudo ufw status verbose