When creating an image fails because the format doesn't support option
"backing_file" or "backing_fmt", bdrv_img_create() first has
qemu_opt_set() put a generic error into @local_err, then puts the real
error into @errp with error_setg(), and then propagates the former to
the latter, which throws away the generic error.  A bit complicated,
but works.

Now that qemu_opt_set() returns a useful value, we can simply ignore
the generic error instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200707160613.848843-16-armbru@redhat.com>
---
 block.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 8d478bdc51..b396f32a51 100644
--- a/block.c
+++ b/block.c
@@ -6090,7 +6090,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
 
     if (base_filename) {
         if (!qemu_opt_set(opts, BLOCK_OPT_BACKING_FILE, base_filename,
-                          &local_err)) {
+                          NULL)) {
             error_setg(errp, "Backing file not supported for file format '%s'",
                        fmt);
             goto out;
@@ -6098,7 +6098,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
     }
 
     if (base_fmt) {
-        if (!qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, base_fmt, &local_err)) {
+        if (!qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, base_fmt, NULL)) {
             error_setg(errp, "Backing file format not supported for file "
                              "format '%s'", fmt);
             goto out;
-- 
2.26.2