block: qcow: Switch parse_qcow and BackingFile::new to BlockResult

Switch parse_qcow and BackingFile::new from qcow::Result to
BlockResult. Every early return site now produces an explicit
BlockError with the appropriate kind. Remaining internal calls to
functions still on qcow::Result rely on the From scaffolding and
will be converted in subsequent commits.

Two helpers are added to BlockError. with_kind replaces the
classification on an existing error, used in QcowDiskSync::new to
avoid double wrapping when the caller needs a different kind.
into_source consumes the error and returns the boxed source, used
at the recursive BackingFile open to extract the qcow::Error for
BackingFileOpen without letting qcow::Error hold a BlockError.

The qcow_sync boundary is simplified to a single closure that
operates on the BlockError already returned by parse_qcow.

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

block/src/error.rs

@@ -167,6 +167,12 @@ impl BlockError {
         self
     }
 
+    /// Replace the error classification (builder-style).
+    pub fn with_kind(mut self, kind: BlockErrorKind) -> Self {
+        self.kind = kind;
+        self
+    }
+
     /// Shorthand: attach an operation name.
     pub fn with_op(mut self, op: ErrorOp) -> Self {
         self.ctx.get_or_insert_with(ErrorContext::default).op = Some(op);
@@ -204,6 +210,11 @@ impl BlockError {
     pub fn downcast_ref<T: StdError + 'static>(&self) -> Option<&T> {
         self.source.as_ref()?.downcast_ref::<T>()
     }
+
+    /// Consume the error and return the boxed source, if any.
+    pub fn into_source(self) -> Option<Box<dyn StdError + Send + Sync + 'static>> {
+        self.source
+    }
 }
 
 impl Display for BlockError {