templatized handling of graphics and video libvirt XML

Adding in template code which lets us completely customize the video/graphics configuration of vagrant-libvirt managed VMs.  This is useful for anyone running multiple GUI-based VMs, or in general with a GUI-based VM.

For example, with QEMU as the hypervisor, SPICE is a more attractive interface than VNC.

This fixes #238, and goes a bit further in templatizing more things.
Actually, would you prefer logic be in the template or in the Ruby code (just put a variable there)?