Commit 21373feb added support for primary virtio-vga device but it was
checking for virtio-gpu. Let's check for existence of virtio-vga if we
want to use it.
Virtio video device is currently represented by three different models
*virtio-gpu-device*, *virtio-gpu-pci* and *virtio-vga*. The first two
models are tied together and if virtio video devices is compiled in they
both exist. However, the *virtio-vga* model doesn't have to exist on
some architectures even if the first two models exist. So we cannot
group all three together.
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>