rustc: Allow Descriptor and VirtioRng to be unused

Rust v1.89.0 is complaining about VirtioRng, VirtioSnd and Descriptor
being unused, remove VirtioRng, VirtioSnd and add #[allow(unused)] to
suppress those warnings.

```console
warning: struct `Descriptor` is never constructed
   --> src/rutabaga_gfx/src/rutabaga_os/descriptor.rs:143:12
    |
143 | pub struct Descriptor(pub RawDescriptor);
    |            ^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default
```

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

Author: RuoqingHe

src/devices/src/virtio/rng/device.rs

@@ -14,10 +14,6 @@ pub(crate) const REQ_INDEX: usize = 0;
 // Supported features.
 pub(crate) const AVAIL_FEATURES: u64 = 1 << uapi::VIRTIO_F_VERSION_1 as u64;
 
-#[derive(Copy, Clone, Debug, Default)]
-#[repr(C, packed)]
-pub struct VirtioRng {}
-
 pub struct Rng {
     pub(crate) queues: Vec<VirtQueue>,
     pub(crate) queue_events: Vec<EventFd>,

src/devices/src/virtio/snd/device.rs

@@ -15,10 +15,6 @@ use crate::virtio::{DeviceState, InterruptTransport};
 // Supported features.
 pub(crate) const AVAIL_FEATURES: u64 = 1 << uapi::VIRTIO_F_VERSION_1 as u64;
 
-#[derive(Copy, Clone, Debug, Default)]
-#[repr(C, packed)]
-pub struct VirtioSnd {}
-
 pub struct Snd {
     pub(crate) queues: Vec<VirtQueue>,
     pub(crate) queue_events: Vec<EventFd>,

src/rutabaga_gfx/src/rutabaga_os/descriptor.rs

@@ -138,6 +138,7 @@ impl From<File> for SafeDescriptor {
 ///
 /// Note that with the exception of the last use-case (which requires proper error checking against
 /// the descriptor being closed), the `Descriptor` instance would be very short-lived.
+#[allow(unused)]
 #[derive(Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
 #[repr(transparent)]
 pub struct Descriptor(pub RawDescriptor);