OvmfPkg/PlatformBootManagerLib: relax device class requirement for ConOut

OvmfPkg/PlatformBootManagerLib: relax device class requirement for ConOut

This will add virtio-gpu-pci devices to ConOut automatically.

For further benefit, the change also allows OVMF to use the legacy-free /
secondary VGA adapter (added in QEMU commit 63e3e24d, "vga: add secondary
stdvga variant") as console.

ArmVirtPkg's PlatformBootManagerLib already filters with IS_PCI_DISPLAY();
see IsPciDisplay().

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
Originally-suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>