conf: Introduce new video type 'none'

conf: Introduce new video type 'none'

Historically, we've always enabled an emulated video device every time we
see that graphics should be supported with a guest. With the appearance
of mediated devices which can support QEMU's vfio-display capability,
users might want to use such a device as the only video device.
Therefore introduce a new, effectively a 'disable', type for video
device.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Erik Skultety <eskultet@redhat.com>