ArmVirtPkg: resolve QemuFwCfgS3Lib
    
    QemuFwCfgS3Enabled() in "ArmVirtPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c"
    returns constant FALSE.
    
    The same implementation is now available factored-out in
    "OvmfPkg/Library/QemuFwCfgS3Lib/QemuFwCfgS3Base.c".
    
    Resolve QemuFwCfgS3Lib to BaseQemuFwCfgS3LibNull.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=394
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>