OvmfPkg: resolve QemuFwCfgS3Lib

QemuFwCfgS3Enabled() in "OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c"
queries the "etc/system-states" fw_cfg file.

The same implementation is now available factored-out in
"OvmfPkg/Library/QemuFwCfgS3Lib/QemuFwCfgS3PeiDxe.c". It is available to
PEIMs through the PeiQemuFwCfgS3LibFwCfg instance, and to DXE_DRIVER and
DXE_RUNTIME_DRIVER modules through the DxeQemuFwCfgS3LibFwCfg instance.

Resolve QemuFwCfgS3Lib accordingly.

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>