OvmfPkg/QemuFwCfgS3Lib: implement opcode APIs for DXE fw_cfg instance

OvmfPkg/QemuFwCfgS3Lib: implement opcode APIs for DXE fw_cfg instance

In the DXE fw_cfg instance:

- QemuFwCfgS3Enabled() queries S3 enablement via fw_cfg. This behavior is
  shared with the PEI fw_cfg instance, and the DXE fw_cfg instance already
  pulls in the function from "QemuFwCfgS3PeiDxe.c".

- If QemuFwCfgS3Enabled() returns TRUE, the client module is permitted to
  call QemuFwCfgS3CallWhenBootScriptReady().

  We provide a fully functional implementation for
  QemuFwCfgS3CallWhenBootScriptReady(). A protocol notify is installed at
  TPL_CALLBACK for EFI_S3_SAVE_STATE_PROTOCOL. If / once the protocol is
  available, the client module's Callback() function is called, which is
  expected to produce ACPI S3 Boot Script opcodes using the helper
  functions listed below. In QemuFwCfgS3CallWhenBootScriptReady(), we also
  allocate a reserved memory buffer, sized & typed by the client module,
  for the opcodes and (internally) the fw_cfg DMA operations to work upon,
  during S3 resume.

  This behavior is unique to the DXE fw_cfg instance. Thus, add the
  function to "QemuFwCfgS3Dxe.c".

- The QemuFwCfgS3ScriptWriteBytes(), QemuFwCfgS3ScriptReadBytes(),
  QemuFwCfgS3ScriptSkipBytes(), and QemuFwCfgS3ScriptCheckValue()
  functions are also implemented usefully, since the client module's
  Callback() function is expected to invoke them.

  Each of the first three functions produces MEM_WRITE, IO_WRITE, and
  MEM_POLL opcodes, to set up the DMA command in reserved memory, to start
  the DMA transfer, and to check the DMA result, respectively.

  The QemuFwCfgS3ScriptCheckValue() function produces a MEM_POLL opcode to
  validate an unsigned integer field in data that was read via
  QemuFwCfgS3ScriptReadBytes().

  This behavior is again unique to the DXE fw_cfg instance, so add the
  functions to "QemuFwCfgS3Dxe.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>