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/igvm/igvm_loader.rs

@@ -129,6 +129,21 @@ fn import_parameter(
     Ok(())
 }
 
+///
+/// Extract sev_features from the boot CPU (vp_index 0) VMSA.
+///
+#[cfg(feature = "sev_snp")]
+pub fn extract_sev_features(igvm_file: &IgvmFile) -> u64 {
+    for header in igvm_file.directives() {
+        if let IgvmDirectiveHeader::SnpVpContext { vp_index, vmsa, .. } = header
+            && *vp_index == 0
+        {
+            return vmsa.sev_features.into();
+        }
+    }
+    0
+}
+
 ///
 /// Load the given IGVM file to guest memory.
 /// Right now it only supports SNP based isolation.