Default graphics type of spice incompatible with RHEL9.

[paisley]

This provider is unable to provision VMs using libvirt running on RHEL9 and its derivatives (CentOS 9, Rocky Linux 9, Alma Linux 9, etc.) due to the default graphics type being set to spice and SPICE support being dropped in newer versions of RHEL. The graphics type of VNC (vnc) is compatible with these versions and shares the same options (listen_type and autoport) as the default spice graphics specified in main.tf.

Recommend changing this to a variable, leaving spice as the default, but also allowing vnc as an option. I will open a pull request with this proposed change shortly.