vhost_user: format links in docs correctly

This resolves a rustdoc warning, and makes the links appear as links
in the rendered documentation.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost/src/vhost_user/gpu_backend_req.rs

@@ -100,7 +100,7 @@ impl BackendInternal {
 
 /// Proxy for sending messages from the backend to the fronted
 /// over the socket obtained from VHOST_USER_GPU_SET_SOCKET.
-/// The protocol is documented here: https://www.qemu.org/docs/master/interop/vhost-user-gpu.html
+/// The protocol is documented here: <https://www.qemu.org/docs/master/interop/vhost-user-gpu.html>
 #[derive(Clone)]
 pub struct GpuBackend {
     // underlying Unix domain socket for communication

vhost/src/vhost_user/gpu_message.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 //! Implementation parts of the protocol on the socket from VHOST_USER_SET_GPU_SOCKET
-//! see: https://www.qemu.org/docs/master/interop/vhost-user-gpu.html
+//! see: <https://www.qemu.org/docs/master/interop/vhost-user-gpu.html>
 
 use super::enum_value;
 use crate::vhost_user::message::{MsgHeader, Req, VhostUserMsgValidator};