block: expect errors from bdrv_co_is_allocated

Some bdrv_is_allocated callers do not expect errors, but the fallback
in qcow2.c might make other callers trip on assertion failures or
infinite loops.

Fix the callers to always look for errors.

Cc: qemu-stable@nongnu.org
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/qemu-img.c b/qemu-img.c
index 744c0d9..27cc006 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1508,8 +1508,15 @@
                    are present in both the output's and input's base images (no
                    need to copy them). */
                 if (out_baseimg) {
-                    if (!bdrv_is_allocated(bs[bs_i], sector_num - bs_offset,
-                                           n, &n1)) {
+                    ret = bdrv_is_allocated(bs[bs_i], sector_num - bs_offset,
+                                            n, &n1);
+                    if (ret < 0) {
+                        error_report("error while reading metadata for sector "
+                                     "%" PRId64 ": %s",
+                                     sector_num - bs_offset, strerror(-ret));
+                        goto out;
+                    }
+                    if (!ret) {
                         sector_num += n1;
                         continue;
                     }
@@ -2099,6 +2106,11 @@
 
             /* If the cluster is allocated, we don't need to take action */
             ret = bdrv_is_allocated(bs, sector, n, &n);
+            if (ret < 0) {
+                error_report("error while reading image metadata: %s",
+                             strerror(-ret));
+                goto out;
+            }
             if (ret) {
                 continue;
             }