The uuid generation doesn't return error, so update the function
signature and calling code accordingly.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-7-git-send-email-famz@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>