OvmfPkg: introduce QemuFwCfgS3Lib class

OvmfPkg: introduce QemuFwCfgS3Lib class

This library class will enable driver modules (a) to query whether S3
support was enabled on the QEMU command line, (b) to produce fw_cfg DMA
operations that are to be replayed at S3 resume time.

Declare the library class in OvmfPkg/OvmfPkg.dec, and add the library
class header under OvmfPkg/Include/Library/. At the moment, the only API
we expose is QemuFwCfgS3Enabled(), which we'll first migrate from
QemuFwCfgLib. Further interfaces will be added in later patches.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=394
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>