riscv64: Enable set_device_attr doc test

Originally `set_device_attr` uses VFIO device for testing, but that's
not available yet on RISC-V 64-bit platform, the QEMU patch [1] and
kernel patch [2] are not upstreamed. Disabling VFIO device test for
RISC-V at the time being.

[1] https://lore.kernel.org/all/[email protected]/
[2] https://github.com/ventanamicro/linux/tree/dev-upstream

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

Author: RuoqingHe

src/ioctls/device.rs

@@ -41,36 +41,46 @@ impl DeviceFd {
     ///
     /// # Example
     ///
+    /// Configuring a VFIO device using `set_device_attr`. Note that VFIO
+    /// devices are not yet available on RISC-V The patch for QEMU:
+    /// https://lore.kernel.org/all/[email protected]/
+    /// and patch for linux kernel
+    /// https://github.com/ventanamicro/linux/tree/dev-upstream are both not
+    /// upstreamed. Disabling VFIO device test for RISC-V at the time being.
+    ///
     /// ```rust
     /// # extern crate kvm_ioctls;
     /// # extern crate kvm_bindings;
     /// # use kvm_ioctls::Kvm;
-    /// # use kvm_bindings::{
-    ///    kvm_device_type_KVM_DEV_TYPE_VFIO,
-    ///    KVM_DEV_VFIO_GROUP, KVM_DEV_VFIO_GROUP_ADD, KVM_CREATE_DEVICE_TEST
-    /// };
     /// let kvm = Kvm::new().unwrap();
     /// let vm = kvm.create_vm().unwrap();
     ///
-    /// let mut device = kvm_bindings::kvm_create_device {
-    ///     type_: kvm_device_type_KVM_DEV_TYPE_VFIO,
-    ///     fd: 0,
-    ///     flags: KVM_CREATE_DEVICE_TEST,
-    /// };
+    /// #[cfg(not(target_arch = "riscv64"))]
+    /// {
+    ///     # use kvm_bindings::{
+    ///     #     kvm_device_type_KVM_DEV_TYPE_VFIO,
+    ///     #     KVM_DEV_VFIO_GROUP, KVM_DEV_VFIO_GROUP_ADD, KVM_CREATE_DEVICE_TEST
+    ///     # };
+    ///     let mut device = kvm_bindings::kvm_create_device {
+    ///         type_: kvm_device_type_KVM_DEV_TYPE_VFIO,
+    ///         fd: 0,
+    ///         flags: KVM_CREATE_DEVICE_TEST,
+    ///     };
     ///
-    /// let device_fd = vm
-    ///     .create_device(&mut device)
-    ///     .expect("Cannot create KVM device");
+    ///     let device_fd = vm
+    ///         .create_device(&mut device)
+    ///         .expect("Cannot create KVM device");
     ///
-    /// let dist_attr = kvm_bindings::kvm_device_attr {
-    ///     group: KVM_DEV_VFIO_GROUP,
-    ///     attr: u64::from(KVM_DEV_VFIO_GROUP_ADD),
-    ///     addr: 0x0,
-    ///     flags: 0,
-    /// };
+    ///     let dist_attr = kvm_bindings::kvm_device_attr {
+    ///         group: KVM_DEV_VFIO_GROUP,
+    ///         attr: u64::from(KVM_DEV_VFIO_GROUP_ADD),
+    ///         addr: 0x0,
+    ///         flags: 0,
+    ///     };
     ///
-    /// if (device_fd.has_device_attr(&dist_attr).is_ok()) {
-    ///     device_fd.set_device_attr(&dist_attr).unwrap();
+    ///     if (device_fd.has_device_attr(&dist_attr).is_ok()) {
+    ///         device_fd.set_device_attr(&dist_attr).unwrap();
+    ///     }
     /// }
     /// ```
     pub fn set_device_attr(&self, device_attr: &kvm_device_attr) -> Result<()> {