OvmfPkg/QemuFwCfgS3Lib: implement opcode APIs for Base Null instance

In the Base Null instance:

- QemuFwCfgS3Enabled() returns constant FALSE. This is unique to the Base
  Null instance, and the function is already present in
  "QemuFwCfgS3Base.c".

- The QemuFwCfgS3CallWhenBootScriptReady() function must never be called
  (according to the documentation, given the above). This is also unique
  to the Base Null instance, so implement the function in
  "QemuFwCfgS3Base.c".

- Consequently, the QemuFwCfgS3ScriptWriteBytes(),
  QemuFwCfgS3ScriptReadBytes(), QemuFwCfgS3ScriptSkipBytes(), and
  QemuFwCfgS3ScriptCheckValue() functions must never be called either.
  This behavior is not unique to the Base Null instance (it will be shared
  with the PEI fw_cfg instance), so add these functions to
  "QemuFwCfgS3BasePei.c".

Cc: Jordan Justen <jordan.l.justen@intel.com>
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>

OvmfPkg/Library/QemuFwCfgS3Lib/QemuFwCfgS3Base.c

@@ -16,6 +16,7 @@
   WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 **/
 
+#include <Library/DebugLib.h>
 #include <Library/QemuFwCfgS3Lib.h>
 
 /**
@@ -37,3 +38,73 @@ QemuFwCfgS3Enabled (
 {
   return FALSE;
 }
+
+
+/**
+  Install the client module's FW_CFG_BOOT_SCRIPT_CALLBACK_FUNCTION callback for
+  when the production of ACPI S3 Boot Script opcodes becomes possible.
+
+  Take ownership of the client-provided Context, and pass it to the callback
+  function, when the latter is invoked.
+
+  Allocate scratch space for those ACPI S3 Boot Script opcodes to work upon
+  that the client will produce in the callback function.
+
+  @param[in] Callback           FW_CFG_BOOT_SCRIPT_CALLBACK_FUNCTION to invoke
+                                when the production of ACPI S3 Boot Script
+                                opcodes becomes possible. Callback() may be
+                                called immediately from
+                                QemuFwCfgS3CallWhenBootScriptReady().
+
+  @param[in,out] Context        Client-provided data structure for the
+                                Callback() callback function to consume.
+
+                                If Context points to dynamically allocated
+                                memory, then Callback() must release it.
+
+                                If Context points to dynamically allocated
+                                memory, and
+                                QemuFwCfgS3CallWhenBootScriptReady() returns
+                                successfully, then the caller of
+                                QemuFwCfgS3CallWhenBootScriptReady() must
+                                neither dereference nor even evaluate Context
+                                any longer, as ownership of the referenced area
+                                has been transferred to Callback().
+
+  @param[in] ScratchBufferSize  The size of the scratch buffer that will hold,
+                                in reserved memory, all client data read,
+                                written, and checked by the ACPI S3 Boot Script
+                                opcodes produced by Callback().
+
+  @retval RETURN_UNSUPPORTED       The library instance does not support this
+                                   function.
+
+  @retval RETURN_NOT_FOUND         The fw_cfg DMA interface to QEMU is
+                                   unavailable.
+
+  @retval RETURN_BAD_BUFFER_SIZE   ScratchBufferSize is too large.
+
+  @retval RETURN_OUT_OF_RESOURCES  Memory allocation failed.
+
+  @retval RETURN_SUCCESS           Callback() has been installed, and the
+                                   ownership of Context has been transferred.
+                                   Reserved memory has been allocated for the
+                                   scratch buffer.
+
+                                   A successful invocation of
+                                   QemuFwCfgS3CallWhenBootScriptReady() cannot
+                                   be rolled back.
+
+  @return                          Error codes from underlying functions.
+**/
+EFIAPI
+RETURN_STATUS
+QemuFwCfgS3CallWhenBootScriptReady (
+  IN     FW_CFG_BOOT_SCRIPT_CALLBACK_FUNCTION *Callback,
+  IN OUT VOID                                 *Context,          OPTIONAL
+  IN     UINTN                                ScratchBufferSize
+  )
+{
+  ASSERT (FALSE);
+  return RETURN_UNSUPPORTED;
+}