virtio: Enable qcow support for virtio-block

With this enabled, one can pass a QCOW format disk
image with '--disk' switch.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>

qcow/src/refcount.rs

@@ -48,7 +48,7 @@ impl Display for Error {
 }
 
 /// Represents the refcount entries for an open qcow file.
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct RefCount {
     ref_table: VecCache<u64>,
     refcount_table_offset: u64,