OvmfPkg/AmdSev/SecretPei: Mark SEV launch secret area as reserved

OvmfPkg/AmdSev/SecretPei: Mark SEV launch secret area as reserved

Mark the SEV launch secret MEMFD area as reserved, which will allow the
guest OS to use it during the lifetime of the OS, without creating
copies of the sensitive content.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Tobin Feldman-Fitzthum <tobin@linux.ibm.com>
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.Yao@intel.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>