OvmfPkg/QemuVideoDxe: don't incorrectly bind virtio-gpu-pci

OvmfPkg/QemuVideoDxe: don't incorrectly bind virtio-gpu-pci

The PCI (Vendor ID, Device ID) pair (0x1af4, 0x1050) stands for both the
virtio-vga and the virtio-gpu-pci device models of QEMU. They differ in
two things:

- the former has a VGA-compatibility linear framebuffer on top of the
  latter,

- the former has PCI_CLASS_DISPLAY_VGA device class, while the latter has
  PCI_CLASS_DISPLAY_OTHER.

In commit 94210dc95e9f ("OvmfPkg: QemuVideoDxe: add virtio-vga support"),
we enabled QemuVideoDxe to drive virtio-vga simply by adding its (Vendor
ID, Device ID) pair to gQemuVideoCardList. This change inadvertently
allowed QemuVideoDxe to bind virtio-gpu-pci, which it cannot drive though.

Restrict QemuVideoDxe to PCI_CLASS_DISPLAY_VGA, in order to exclude
virtio-gpu-pci. For the other cards that QemuVideoDxe drives, this makes
no difference. (Note that OvmfPkg's PlatformBootManagerLib instance has
always only added PCI_CLASS_DISPLAY_VGA devices to ConOut; see
DetectAndPreparePlatformPciDevicePath().)

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
Fixes: 94210dc95e9f7c6ff4066a9b35a288e6f1c271bf
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>