OvmfPkg/QemuFwCfgS3Lib: add boot script opcode generation APIs to libclass

OvmfPkg/QemuFwCfgS3Lib: add boot script opcode generation APIs to libclass

Introduce the following APIs:

- QemuFwCfgS3CallWhenBootScriptReady(): central function that registers a
  callback function, with a context parameter, for when ACPI S3 Boot
  Script opcodes can be produced. This function also allocates reserved
  memory for the opcodes to operate upon.

  The client module is supposed to produce the boot script fragment in the
  callback function.

- QemuFwCfgS3ScriptWriteBytes(), QemuFwCfgS3ScriptReadBytes(),
  QemuFwCfgS3ScriptSkipBytes(), QemuFwCfgS3ScriptCheckValue(): helper
  functions, available only to the above callback function, for composing
  the boot script fragment. QemuFwCfgS3ScriptSkipBytes() can double as a
  plain "select" whenever necessary.

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>