OvmfPkg: include VirtioGpuDxe in the platform DSC/FDF files

OvmfPkg: include VirtioGpuDxe in the platform DSC/FDF files

At this stage, the driver builds, and suffices for testing binding and
unbinding.

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>