OvmfPkg/PlatformPei: set PcdConfidentialComputingAttr when SEV is active

OvmfPkg/PlatformPei: set PcdConfidentialComputingAttr when SEV is active

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

The MpInitLib uses the ConfidentialComputingAttr PCD to determine whether
AMD SEV is active so that it can use the VMGEXITs defined in the GHCB
specification to create APs.

Cc: Michael Roth <michael.roth@amd.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Suggested-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>