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>