The block commit API checked 'disk->src->path' to see whether there
is a reasonable disk source to be committed. As the top image can be
e.g. backed by NBD the check is not good enough. Replace it by
virStorageSourceIsEmpty.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>