Switch over to virtio_instance_init_common.  Drop duplicate properties
in virtio-gpu-pci and virtio-vga as they are properly aliased now.  Also
drop the indirection via DEFINE_VIRTIO_GPU_PROPERTIES, we don't need it
any more as the properties are defined in a single place now.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>