Network slicing has been a major selling point of 5G networks, where a slice is roughly defined as a self-contained logical network on top of a shared infrastructure tailored for a specific service or vertical industry, composed of heterogeneous resources (computing, storage, bandwidths) and typically involving specific application and/or networking functions. The relationship between the network slicing and the underlying network virtualization is an open research topic, allowing multiple deployment models. In particular, a specific model of interest involves the virtualization of the optical infrastructure, where optical virtual networks are instantiated in support of network slicing, so the virtual network underlies and provides connectivity to component functions of a network slice. In this paper, we propose and implement a network virtualization architecture for open optical (partially) disaggregated networks, based on the concept of a device hypervisor relying on OpenROADM data models, in support of 5G network slicing over interconnected network function virtualization infrastructure points of presence (NFVI-PoPs). The architecture is experimentally validated, showing the provisioning of ITU-T flexigrid network media channels across a virtualized network.