As of the patch to flush qemu-img's "Formatting" message before the
error message, 059 has been broken for vmdk.  Fix it.

Fixes: 4e2f4418
       ("qemu-img: Flush stdout before before potential stderr messages")
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200811084150.326377-1-mreitz@redhat.com>
Reviewed-by: Eric blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>