Add none option to graphics

[flx5]

Add an option to remove the graphics device from the domain definition.

Motivation:
The minimal ubuntu cloud server images do only print to serial console anyways.
To avoid confusion (the SPICE Console only shows attempting initrdless boot which could easily be interpreted as the OS not booting), I wanted to disable the spice console so that virt-manager always shows the serial console directly.

Considered options:

• Remove graphics in default and require user to explicitly add graphics if wanted.
• Add none to graphics device type to disable graphics.

I chose to implement the second option because the first option would be a major breaking change.

See also #359