hypervisor, vmm: Add KVM SEV_{INIT2, SNP_LAUNCH_START} support

Introduce the SevFd abstraction that wraps /dev/sev and implements the
KVM_SEV_INIT2 and KVM_SEV_SNP_LAUNCH_START ioctls for SEV-SNP VM
initialization on KVM.

Key changes:
- Add sev.rs with KvmSevInit and KvmSevSnpLaunchStart ioctl structs
  matching the kernel layout (linux/arch/x86/include/uapi/asm/kvm.h)
- Implement KVM_SEV_INIT2 and KVM_SEV_SNP_LAUNCH_START ioctls
- Set KVM_MEMORY_ATTRIBUTE_PRIVATE on newly created memory regions
  when guest_memfd is supported
- Widen SevSnpPageAccessProxy cfg gates from mshv-only to all
  sev_snp-enabled builds
- Make sev_snp_init a required trait method (remove default impl)
- Include KVM_SEV_SNP_LAUNCH_START in the seccomp allowlist
- Parse VMSA SEV features from IGVM and include them in the
  KVM_SEV_INIT2 ioctl

Co-authored-by: Keith Adler <kadler@cloudflare.com>
Signed-off-by: Keith Adler <kadler@cloudflare.com>
Co-authored-by: Alex Orozco <aorozco@google.com>
Signed-off-by: Alex Orozco <aorozco@google.com>
Co-authored-by: Rob Bradford <rbradford@meta.com>
Signed-off-by: Rob Bradford <rbradford@meta.com>
Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/src/vm.rs

@@ -1327,10 +1327,16 @@ impl Vm {
                 .map_err(Error::IgvmLoad)?
         };
 
-        let vm = Self::create_hypervisor_vm(
-            hypervisor.as_ref(),
-            vm_config.as_ref().lock().unwrap().deref().into(),
-        )?;
+        let vm = {
+            #[allow(unused_mut)]
+            let mut hv_config: hypervisor::HypervisorVmConfig =
+                vm_config.as_ref().lock().unwrap().deref().into();
+            #[cfg(all(feature = "igvm", feature = "sev_snp"))]
+            if let Some(ref igvm) = igvm_file {
+                hv_config.vmsa_features = igvm_loader::extract_sev_features(igvm);
+            }
+            Self::create_hypervisor_vm(hypervisor.as_ref(), hv_config)?
+        };
 
         #[cfg(all(feature = "kvm", target_arch = "x86_64"))]
         if vm_config.lock().unwrap().max_apic_id() > MAX_SUPPORTED_CPUS_LEGACY {