hypervisor, vmm: Build and pass SNP ID block to launch finish

Add KvmSevSnpIdBlock and KvmSevSnpIdAuth structs matching the AMD
SEV-SNP Firmware ABI Spec (Rev 1.58), and build them from the IGVM
SNP ID block directive during launch finish. This properly populates
id_block_uaddr/id_auth_uaddr in KVM_SEV_SNP_LAUNCH_FINISH and derives
auth_key_en from the assembled author key, matching QEMU's behavior.

Thread the guest policy from sev_snp_init to launch_finish via an
atomic on KvmVm so the ID block gets the correct policy value.

Also track has_snp_id_block in IgvmLoadedInfo to enable the ID block
based on whether the IGVM file actually contains one, rather than
hardcoding it for KVM.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

hypervisor/src/mshv/mod.rs

@@ -2318,6 +2318,7 @@ impl vm::Vm for MshvVm {
         snp_id_block: IGVM_VHS_SNP_ID_BLOCK,
         host_data: [u8; 32],
         id_block_enabled: u8,
+        auth_key_enabled: u8,
     ) -> vm::Result<()> {
         let mut auth_info = hv_snp_id_auth_info {
             id_key_algorithm: snp_id_block.id_key_algorithm,
@@ -2351,7 +2352,7 @@ impl vm::Vm for MshvVm {
                     id_auth_info: auth_info,
                     host_data,
                     id_block_enabled,
-                    author_key_enabled: 0,
+                    author_key_enabled: auth_key_enabled,
                 },
             },
         };