block: disk_file: Add SparseCapable trait

Sparse and zero flag support for thin provisioned disk
images. Two methods with false defaults: sparse operations
(punch hole, write zeroes, discard) and zero flag
optimization in WRITE_ZEROES.

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

block/src/disk_file.rs

@@ -65,3 +65,19 @@ pub trait Geometry: Send + Debug {
         DiskTopology::default()
     }
 }
+
+/// Sparse and zero flag support for thin provisioned disk images.
+pub trait SparseCapable: Send + Debug {
+    /// Indicates support for sparse operations (punch hole, write zeroes, discard).
+    fn supports_sparse_operations(&self) -> bool {
+        false
+    }
+
+    /// Indicates support for a metadata level zero flag optimization in
+    /// virtio `VIRTIO_BLK_T_WRITE_ZEROES` requests. When true, the format
+    /// can mark regions as reading zeros via a metadata bit rather than
+    /// writing actual zero bytes to disk.
+    fn supports_zero_flag(&self) -> bool {
+        false
+    }
+}