commit: Implement .bdrv_refresh_filename

We want query-block to return the right filename, even if a commit job
put a bdrv_commit_top on top of the actual image format driver. Let
bdrv_commit_top.bdrv_refresh_filename get the filename from its backing
file.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block/commit.c

@@ -13,6 +13,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/cutils.h"
 #include "trace.h"
 #include "block/block_int.h"
 #include "block/blockjob_int.h"
@@ -242,6 +243,12 @@ static int64_t coroutine_fn bdrv_commit_top_get_block_status(
            (sector_num << BDRV_SECTOR_BITS);
 }
 
+static void bdrv_commit_top_refresh_filename(BlockDriverState *bs, QDict *opts)
+{
+    bdrv_refresh_filename(bs->backing->bs);
+    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
+            bs->backing->bs->filename);
+}
 
 static void bdrv_commit_top_close(BlockDriverState *bs)
 {
@@ -262,6 +269,7 @@ static BlockDriver bdrv_commit_top = {
     .format_name                = "commit_top",
     .bdrv_co_preadv             = bdrv_commit_top_preadv,
     .bdrv_co_get_block_status   = bdrv_commit_top_get_block_status,
+    .bdrv_refresh_filename      = bdrv_commit_top_refresh_filename,
     .bdrv_close                 = bdrv_commit_top_close,
     .bdrv_child_perm            = bdrv_commit_top_child_perm,
 };