block: Improve get_allocated_file_size's default

There are two practical problems with bdrv_get_allocated_file_size()'s
default right now:
(1) For drivers with children, we should generally sum all their sizes
    instead of just passing the request through to bs->file.  The latter
    is good for filters, but not so much for format drivers.

(2) Filters need not have bs->file, so we should actually go to the
    filtered child instead of hard-coding bs->file.

Fix this by splitting the default implementation into three branches:
(1) For filter drivers: Return the size of the filtered child
(2) For protocol drivers: Return -ENOTSUP, because the default
    implementation cannot make a guess
(3) For other drivers: Sum all data-bearing children's sizes

Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -5005,6 +5005,31 @@ exit:
     return ret;
 }
 
+/**
+ * Implementation of BlockDriver.bdrv_get_allocated_file_size() that
+ * sums the size of all data-bearing children.  (This excludes backing
+ * children.)
+ */
+static int64_t bdrv_sum_allocated_file_size(BlockDriverState *bs)
+{
+    BdrvChild *child;
+    int64_t child_size, sum = 0;
+
+    QLIST_FOREACH(child, &bs->children, next) {
+        if (child->role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
+                           BDRV_CHILD_FILTERED))
+        {
+            child_size = bdrv_get_allocated_file_size(child->bs);
+            if (child_size < 0) {
+                return child_size;
+            }
+            sum += child_size;
+        }
+    }
+
+    return sum;
+}
+
 /**
  * Length of a allocated file in bytes. Sparse files are counted by actual
  * allocated space. Return < 0 if error or unknown.
@@ -5018,10 +5043,21 @@ int64_t bdrv_get_allocated_file_size(BlockDriverState *bs)
     if (drv->bdrv_get_allocated_file_size) {
         return drv->bdrv_get_allocated_file_size(bs);
     }
-    if (bs->file) {
-        return bdrv_get_allocated_file_size(bs->file->bs);
-    }
+
+    if (drv->bdrv_file_open) {
+        /*
+         * Protocol drivers default to -ENOTSUP (most of their data is
+         * not stored in any of their children (if they even have any),
+         * so there is no generic way to figure it out).
+         */
         return -ENOTSUP;
+    } else if (drv->is_filter) {
+        /* Filter drivers default to the size of their filtered child */
+        return bdrv_get_allocated_file_size(bdrv_filter_bs(bs));
+    } else {
+        /* Other drivers default to summing their children's sizes */
+        return bdrv_sum_allocated_file_size(bs);
+    }
 }
 
 /*