qapi: Tweak error message of bdrv_query_image_info

@bs doesn't always have a device name, such as when it comes from
"qemu-img info". Report file name instead.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 20170119130759.28319-2-famz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>