block: add bdrv_find_backing_image

Add bdrv_find_backing_image: given a BlockDriverState pointer, and an id,
traverse the backing image chain to locate the id.
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -2595,6 +2595,24 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs,
     return -ENOTSUP;
 }
 
+BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
+        const char *backing_file)
+{
+    if (!bs->drv) {
+        return NULL;
+    }
+
+    if (bs->backing_hd) {
+        if (strcmp(bs->backing_file, backing_file) == 0) {
+            return bs->backing_hd;
+        } else {
+            return bdrv_find_backing_image(bs->backing_hd, backing_file);
+        }
+    }
+
+    return NULL;
+}
+
 #define NB_SUFFIXES 4
 
 char *get_human_readable_size(char *buf, int buf_size, int64_t size)

block.h

@@ -148,6 +148,8 @@ int coroutine_fn bdrv_co_writev(BlockDriverState *bs, int64_t sector_num,
     int nb_sectors, QEMUIOVector *qiov);
 int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t sector_num,
     int nb_sectors, int *pnum);
+BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
+    const char *backing_file);
 int bdrv_truncate(BlockDriverState *bs, int64_t offset);
 int64_t bdrv_getlength(BlockDriverState *bs);
 int64_t bdrv_get_allocated_file_size(BlockDriverState *bs);