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/vm.rs

@@ -440,6 +440,7 @@ pub trait Vm: Send + Sync + Any {
         _snp_id_block: IGVM_VHS_SNP_ID_BLOCK,
         _host_data: [u8; 32],
         _id_block_enabled: u8,
+        _auth_key_enabled: u8,
     ) -> Result<()> {
         unimplemented!()
     }