OvmfPkg/VirtioGpuDxe: implement EFI_GRAPHICS_OUTPUT_PROTOCOL

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.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://tianocore.acgmultimedia.com/show_bug.cgi?id=66
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>