OvmfPkg/QemuFwCfgS3Lib: add initial PEI and DXE fw_cfg library instances

This patch introduces PeiQemuFwCfgS3LibFwCfg, a limited functionality
QemuFwCfgS3Lib instance, for PEI phase modules.

The patch also introduces DxeQemuFwCfgS3LibFwCfg, a full functionality
QemuFwCfgS3Lib instance, for DXE_DRIVER and DXE_RUNTIME_DRIVER modules.

These library instances share the QemuFwCfgS3Enabled() function. The
function actually uses fw_cfg; the implementation is copied from
"OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c".

The library instances will diverge in the following patches.

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/QemuFwCfgS3PeiDxe.c

@@ -0,0 +1,48 @@
+/** @file
+  Shared code for the PEI fw_cfg and DXE fw_cfg instances of the QemuFwCfgS3Lib
+  class.
+
+  Copyright (C) 2017, Red Hat, Inc.
+
+  This program and the accompanying materials are licensed and made available
+  under the terms and conditions of the BSD License which accompanies this
+  distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT
+  WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+**/
+
+#include <Library/QemuFwCfgLib.h>
+#include <Library/QemuFwCfgS3Lib.h>
+
+/**
+  Determine if S3 support is explicitly enabled.
+
+  @retval  TRUE   If S3 support is explicitly enabled. Other functions in this
+                  library may be called (subject to their individual
+                  restrictions).
+
+           FALSE  Otherwise. This includes unavailability of the firmware
+                  configuration interface. No other function in this library
+                  must be called.
+**/
+BOOLEAN
+EFIAPI
+QemuFwCfgS3Enabled (
+  VOID
+  )
+{
+  RETURN_STATUS        Status;
+  FIRMWARE_CONFIG_ITEM FwCfgItem;
+  UINTN                FwCfgSize;
+  UINT8                SystemStates[6];
+
+  Status = QemuFwCfgFindFile ("etc/system-states", &FwCfgItem, &FwCfgSize);
+  if (Status != RETURN_SUCCESS || FwCfgSize != sizeof SystemStates) {
+    return FALSE;
+  }
+  QemuFwCfgSelectItem (FwCfgItem);
+  QemuFwCfgReadBytes (sizeof SystemStates, SystemStates);
+  return (BOOLEAN) (SystemStates[3] & BIT7);
+}