Software-defined networking techniques are useful for implementing a variety of applications and traffic engineering in wide-area networks. Multitenancy in OpenFlow improves the cost-effectiveness of wide-area network infrastructures. The characteristics required for multitenancy in wide-area networks are scalability to the large number of physical OpenFlow switches and applicability to the virtualization of multiple administration networks. Among existing techniques, AutoSlice scales to a large number of OpenFlow switches using distributed proxies, which can be operated by different administrators. However, the programmability of OpenFlow for tenants is limited. Certain header fields (e.g., VLAN tags) are not accessible to tenants, thereby limiting their applications. We propose the AutoVFlow mechanism over distributed proxies to enable flow space virtualization, thus enabling tenants to exercise full control over all header fields. The AutoVFlow mechanism is suitable for the virtualization of multiple administrator networks, because AutoVFlow proxies manage the mapping of header values for virtualization autonomously. We evaluate the overhead of the AutoVFlow mechanism and its scalability to a large number of physical OpenFlow switches. The results show that the overhead is negligible in wide-area OpenFlow networks and that the mechanism can be scaled to the number of physical OpenFlow switches by increasing the number of proxies.