OvmfPkg/PlatformBootManagerLib: use PcdAcpiS3Enable to detect S3 support

OvmfPkg/PlatformBootManagerLib: use PcdAcpiS3Enable to detect S3 support

To avoid the potential inconsistency between PcdAcpiS3Enable and
QemuFwCfgS3Enabled(), this commit modifies PlatformBootManagerLib to
detect S3 support by PcdAcpiS3Enable as modules in MdeModulePkg do.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3573
Signed-off-by: Gary Lin <gary.lin@hpe.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>