OvmfPkg/QemuVideoDxe: enable ARM builds
    
    The only feature not portable to ArmVirtualizationQemu is the VBE shim;
    make that dependent on Ia32 / X64.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>