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>