OvmfPkg/ResetVector: introduce SEV metadata descriptor for VMM use

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

The OvmfPkgX86 build reserves memory regions in MEMFD. The memory regions
get accessed in the SEC phase. AMD SEV-SNP require that the guest's
private memory be accepted or validated before access.

Introduce a Guided metadata structure that describes the reserved memory
regions. The VMM can locate the metadata structure by iterating through
the reset vector guid and process the areas based on the platform
specific requirements.

Cc: Michael Roth <michael.roth@amd.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>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm

@@ -64,6 +64,20 @@ tdxMetadataOffsetStart:
     DB      0x86, 0x5e, 0x46, 0x85, 0xa7, 0xbf, 0x8e, 0xc2
 tdxMetadataOffsetEnd:
 
+;
+; SEV metadata descriptor
+;
+; Provide the start offset of the metadata blob within the OVMF binary.
+
+; GUID : dc886566-984a-4798-A75e-5585a7bf67cc
+;
+OvmfSevMetadataOffsetStart:
+  DD      (fourGigabytes - OvmfSevMetadataGuid)
+  DW      OvmfSevMetadataOffsetEnd - OvmfSevMetadataOffsetStart
+  DB      0x66, 0x65, 0x88, 0xdc, 0x4a, 0x98, 0x98, 0x47
+  DB      0xA7, 0x5e, 0x55, 0x85, 0xa7, 0xbf, 0x67, 0xcc
+OvmfSevMetadataOffsetEnd:
+
 %endif
 
 ; SEV Hash Table Block