OvmfPkg: AcpiS3SaveDxe: call S3Ready() at End-of-Dxe

OvmfPkg: AcpiS3SaveDxe: call S3Ready() at End-of-Dxe

Call S3Ready() whenever the first of the following occurs:
- a driver signals End-of-Dxe,
- a driver calls EFI_ACPI_S3_SAVE_PROTOCOL.S3Save().

S3Ready() already contains a static, function scope "latch" that causes it
to exit early when called for the second time or later.

(At the moment, the only platform in the edk2 tree that includes this
driver is OvmfPkg. That platform does not signal End-of-Dxe (yet).)

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/16088/focus=16146
Suggested-by: Yao Jiewen <jiewen.yao@intel.com>
Cc: 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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18034 6f19259b-4bc3-4df7-8a09-765794883524