riscv64: Disable get_reg_list doc test

Limit `get_reg_list` doc example to run on ARM64 only, since
`get_reg_list` is for ARM64 and RISC-V64 but 500 registers are too big
for RISC-V (a value between 180~200).

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

src/ioctls/vcpu.rs

@@ -1212,14 +1212,17 @@ impl VcpuFd {
     /// let vm = kvm.create_vm().unwrap();
     /// let vcpu = vm.create_vcpu(0).unwrap();
     ///
-    /// // KVM_GET_REG_LIST demands that the vcpus be initalized.
-    /// let mut kvi: kvm_bindings::kvm_vcpu_init = kvm_bindings::kvm_vcpu_init::default();
-    /// vm.get_preferred_target(&mut kvi).unwrap();
-    /// vcpu.vcpu_init(&kvi).expect("Cannot initialize vcpu");
+    /// // KVM_GET_REG_LIST on Aarch64 demands that the vcpus be initialized.
+    /// #[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
+    /// {
+    ///     let mut kvi: kvm_bindings::kvm_vcpu_init = kvm_bindings::kvm_vcpu_init::default();
+    ///     vm.get_preferred_target(&mut kvi).unwrap();
+    ///     vcpu.vcpu_init(&kvi).expect("Cannot initialize vcpu");
     ///
-    /// let mut reg_list = RegList::new(500).unwrap();
-    /// vcpu.get_reg_list(&mut reg_list).unwrap();
-    /// assert!(reg_list.as_fam_struct_ref().n > 0);
+    ///     let mut reg_list = RegList::new(500).unwrap();
+    ///     vcpu.get_reg_list(&mut reg_list).unwrap();
+    ///     assert!(reg_list.as_fam_struct_ref().n > 0);
+    /// }
     /// ```
     #[cfg(any(target_arch = "arm", target_arch = "aarch64", target_arch = "riscv64"))]
     pub fn get_reg_list(&self, reg_list: &mut RegList) -> Result<()> {