In this patch we replace our "dummy" Graphics Output Protocol interface
with the real one. We exploit that EFI_GRAPHICS_OUTPUT_BLT_PIXEL and
VirtioGpuFormatB8G8R8X8Unorm have identical representations; this lets us
forego any pixel format conversions in the guest. For messaging the VirtIo
GPU device, we use the primitives introduced in the previous patch.