build: do not build virtio-vga-gl if virgl/opengl not available If virgl and opengl are not available, the build process creates a useless libvirtio-vga-gl module that does not have any device in it. Follow the example of virtio-vga-rutabaga and do not build the module at all in that case. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>