block: qcow_sync: Add Debug impl for QcowDiskSync

The new composable traits require Debug. Implement it manually since
QcowMetadata contains RwLock state that cannot auto derive.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow_sync.rs

@@ -7,7 +7,7 @@ use std::collections::VecDeque;
 use std::fs::File;
 use std::os::fd::{AsFd, AsRawFd, BorrowedFd, OwnedFd, RawFd};
 use std::sync::Arc;
-use std::{io, ptr, slice};
+use std::{fmt, io, ptr, slice};
 
 use vmm_sys_util::eventfd::EventFd;
 use vmm_sys_util::write_zeroes::{PunchHole, WriteZeroesAt};
@@ -180,6 +180,15 @@ pub struct QcowDiskSync {
     data_raw_file: QcowRawFile,
 }
 
+impl fmt::Debug for QcowDiskSync {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("QcowDiskSync")
+            .field("sparse", &self.sparse)
+            .field("has_backing", &self.backing_file.is_some())
+            .finish_non_exhaustive()
+    }
+}
+
 impl QcowDiskSync {
     pub fn new(
         file: File,