qcow2: flush refcount cache correctly in alloc_refcount_block()

update_refcount() affects the refcount cache, it does not write to disk.
Therefore bdrv_flush(bs->file) does nothing.  We need to flush the
refcount cache in order to write out the refcount updates!

While we're here also add error returns when qcow2_cache_flush() fails.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 9991923b)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/qcow2-refcount.c

@@ -201,7 +201,10 @@ static int alloc_refcount_block(BlockDriverState *bs,
     *refcount_block = NULL;
 
     /* We write to the refcount table, so we might depend on L2 tables */
-    qcow2_cache_flush(bs, s->l2_table_cache);
+    ret = qcow2_cache_flush(bs, s->l2_table_cache);
+    if (ret < 0) {
+        return ret;
+    }
 
     /* Allocate the refcount block itself and mark it as used */
     int64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
@@ -237,7 +240,10 @@ static int alloc_refcount_block(BlockDriverState *bs,
             goto fail_block;
         }
 
-        bdrv_flush(bs->file);
+        ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+        if (ret < 0) {
+            goto fail_block;
+        }
 
         /* Initialize the new refcount block only after updating its refcount,
          * update_refcount uses the refcount cache itself */