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.
Not 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>