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>

hypervisor/src/lib.rs

@@ -190,6 +190,8 @@ pub struct HypervisorVmConfig {
     pub sev_snp_enabled: bool,
     #[cfg(feature = "sev_snp")]
     pub mem_size: u64,
+    #[cfg(feature = "sev_snp")]
+    pub vmsa_features: u64,
     pub nested: bool,
     pub smt_enabled: bool,
 }