block: qcow: Add backing file path to qcow error context

Extend the BackingFileIo and BackingFileOpen variants of
qcow::Error with a path field so that backing file failures
report which file was involved. The path is populated from
the backing file configuration.

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

block/src/qcow_sync.rs

@@ -144,8 +144,11 @@ fn shared_backing_from(bf: BackingFile) -> BlockResult<Arc<dyn BackingRead>> {
 
     let dup_fd = |fd: BorrowedFd<'_>| -> BlockResult<OwnedFd> {
         fd.try_clone_to_owned().map_err(|e| {
-            BlockError::new(BlockErrorKind::Io, QcowError::BackingFileIo(e))
-                .with_op(ErrorOp::DupBackingFd)
+            BlockError::new(
+                BlockErrorKind::Io,
+                QcowError::BackingFileIo(String::new(), e),
+            )
+            .with_op(ErrorOp::DupBackingFd)
         })
     };