block: qcow: Port internal tests to QcowDisk

Port the internal/mod.rs tests from QcowFile to QcowDisk so
the surviving tests exercise the code paths that are hit when used via
QcowDisk (and thus virtio-block).

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Rob Bradford <rbradford@meta.com>

block/src/formats/qcow/internal/metadata.rs

@@ -351,6 +351,24 @@ impl QcowMetadata {
     pub fn decoder(&self) -> Arc<dyn Decoder> {
         Arc::clone(&self.decoder)
     }
+
+    #[cfg(test)]
+    pub fn header(&self) -> QcowHeader {
+        self.inner.read().unwrap().header.clone()
+    }
+
+    #[cfg(test)]
+    pub fn cluster_refcount(&self, address: u64) -> io::Result<u64> {
+        let mut inner = self.inner.write().unwrap();
+        let QcowState {
+            refcounts,
+            raw_file,
+            ..
+        } = &mut *inner;
+        refcounts
+            .get_cluster_refcount(raw_file, address)
+            .map_err(|e| io::Error::other(format!("get_cluster_refcount: {e}")))
+    }
 }
 
 impl QcowState {

block/src/formats/qcow/mod.rs

@@ -123,6 +123,27 @@ impl QcowDisk {
             .unwrap();
         while async_io.next_completed_request().is_some() {}
     }
+
+    /// Synchronous read convenience for tests and benchmarks.
+    #[cfg(test)]
+    pub fn read_all_at(&self, offset: u64, len: usize) -> Vec<u8> {
+        let mut async_io = self.create_async_io(1).unwrap();
+        let mem = Arc::new(GuestMemoryMmap::<()>::from_ranges(&[(GuestAddress(0), len)]).unwrap());
+        let range = [(GuestAddress(0), len as u32)];
+        let target = GuestMemoryTarget::new(Arc::clone(&mem), &range).unwrap();
+        async_io
+            .read_to_memory(offset as libc::off_t, target, 0)
+            .unwrap();
+        while async_io.next_completed_request().is_some() {}
+        let mut buf = vec![0u8; len];
+        mem.read_slice(&mut buf, GuestAddress(0)).unwrap();
+        buf
+    }
+
+    #[cfg(test)]
+    pub(crate) fn metadata(&self) -> &QcowMetadata {
+        &self.metadata
+    }
 }
 
 /// Writes a fresh qcow2 layout into `file`