fix warning: unaligned_references

fix warning, when compiling with 1.53.0
```
warning: reference to packed field is unaligned
   --> src/vhost_user/message.rs:252:53
    |
252 |   unsafe { std::mem::transmute_copy::<u32, R>(&self.request) }
    |                                               ^^^^^^^^^^^^^
    |
    = note: `#[warn(unaligned_references)]` on by default
    = warning: this was previously accepted by the compiler but is being
      phased out; it will become a hard error in a future release!
    = note: for more information, see issue #82523
      <rust-lang/rust#82523>
    = note: fields of packed structs are not properly aligned, and
      creating a misaligned reference is undefined behavior (even if
      that reference is never dereferenced)
```

Signed-off-by: wanglei <[email protected]>

Author: wanglei01

coverage_config_x86_64.json

@@ -1 +1 @@
-{"coverage_score": 82.2, "exclude_path": "src/vhost_kern/", "crate_features": "vhost-user-master,vhost-user-slave"}
+{"coverage_score": 82.3, "exclude_path": "src/vhost_kern/", "crate_features": "vhost-user-master,vhost-user-slave"}

src/vhost_user/message.rs

@@ -223,16 +223,37 @@ bitflags! {
 /// Common message header for vhost-user requests and replies.
 /// A vhost-user message consists of 3 header fields and an optional payload. All numbers are in the
 /// machine native byte order.
-#[allow(safe_packed_borrows)]
 #[repr(packed)]
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Copy)]
 pub(super) struct VhostUserMsgHeader<R: Req> {
     request: u32,
     flags: u32,
     size: u32,
     _r: PhantomData<R>,
 }
 
+impl<R: Req> Debug for VhostUserMsgHeader<R> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Point")
+            .field("request", &{ self.request })
+            .field("flags", &{ self.flags })
+            .field("size", &{ self.size })
+            .finish()
+    }
+}
+
+impl<R: Req> Clone for VhostUserMsgHeader<R> {
+    fn clone(&self) -> VhostUserMsgHeader<R> {
+        *self
+    }
+}
+
+impl<R: Req> PartialEq for VhostUserMsgHeader<R> {
+    fn eq(&self, other: &Self) -> bool {
+        self.request == other.request && self.flags == other.flags && self.size == other.size
+    }
+}
+
 impl<R: Req> VhostUserMsgHeader<R> {
     /// Create a new instance of `VhostUserMsgHeader`.
     pub fn new(request: R, flags: u32, size: u32) -> Self {
@@ -249,7 +270,7 @@ impl<R: Req> VhostUserMsgHeader<R> {
     /// Get message type.
     pub fn get_code(&self) -> R {
         // It's safe because R is marked as repr(u32).
-        unsafe { std::mem::transmute_copy::<u32, R>(&self.request) }
+        unsafe { std::mem::transmute_copy::<u32, R>(&{ self.request }) }
     }
 
     /// Set message type.
@@ -803,7 +824,6 @@ impl DescStateSplit {
 }
 
 /// Inflight I/O queue region for split virtqueues
-#[allow(safe_packed_borrows)]
 #[repr(packed)]
 pub struct QueueRegionSplit {
     /// Features flags of this region
@@ -868,7 +888,6 @@ impl DescStatePacked {
 }
 
 /// Inflight I/O queue region for packed virtqueues
-#[allow(safe_packed_borrows)]
 #[repr(packed)]
 pub struct QueueRegionPacked {
     /// Features flags of this region
@@ -994,7 +1013,10 @@ mod tests {
         hdr.set_version(0x1);
         assert!(hdr.is_valid());
 
+        // Test Debug, Clone, PartiaEq trait
         assert_eq!(hdr, hdr.clone());
+        assert_eq!(hdr.clone().get_code(), hdr.get_code());
+        assert_eq!(format!("{:?}", hdr.clone()), format!("{:?}", hdr));
     }
 
     #[test]