qemu_capabilities: check for existence of virtio-vga

qemu_capabilities: check for existence of virtio-vga

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>