fix(snapshot): arch-gate VcpuEvent::SaveState to x86_64

VcpuState (KVM x86 vcpu state) and Vcpu::save_state/restore_state are already
#[cfg(target_arch = "x86_64")], but the VcpuEvent::SaveState variant that
carries Box<VcpuState> and the running()-state match arm that referenced it
were not — so the vmm failed to compile on aarch64 (E0425: cannot find type
VcpuState). Gate both to x86_64. Snapshot-fork is x86_64-only; aarch64 builds
cold-boot only. Verified: cargo check --target aarch64-unknown-linux-gnu -p vmm
now passes.

Author: RoyLin

src/vmm/src/linux/vstate.rs

@@ -1635,6 +1635,7 @@ impl Vcpu {
                 state = self.exit(FC_EXIT_CODE_GENERIC_ERROR);
             }
             // SaveState only applies while paused; ignore in running state.
+            #[cfg(target_arch = "x86_64")]
             Ok(VcpuEvent::SaveState(_)) => (),
             // All other events or lack thereof have no effect on current 'running' state.
             Err(TryRecvError::Empty) => (),
@@ -1787,6 +1788,8 @@ pub enum VcpuEvent {
     /// Event that should resume the Vcpu.
     Resume,
     /// Save the Vcpu KVM state through the given channel (only valid while paused).
+    /// vCPU KVM-state save/restore is x86_64-only (see [`VcpuState`]).
+    #[cfg(target_arch = "x86_64")]
     SaveState(crossbeam_channel::Sender<Box<VcpuState>>),
     // Serialize and Deserialize to follow after we get the support from kvm-ioctls.
 }