virtio-gpu: build modular

Only build virtio-gpu-device modular (the code which actually depends on
the external virglrenderer library).  virtio-gpu-pci and virtio-vga are
compiled into core qemu still.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20200914134224.29769-7-kraxel@redhat.com