Boxes is an GNOME application in Fedora that is used to create, manage, and run virtual machines, and Google Summer of Code student Adrien Plazas is working on implementing multiple monitor support in Boxes. From Adrien’s Summer of Code submission:

When connecting to a vm that has multiple outputs, show the primary display by default, and offer a ‘monitor switcher’, which could be similar to the workspace switcher in gnome-shell, or could just be a menubutton with previews. Allow to ‘bind’ an external display of the host to a vm in the display configuration. In this case, put up a fullscreen window on the external display showing the secondary output of the vm, while the normal boxes window would continue to show the primary output.

It is still early days for this feature, as evident from a recent update — the Boxes developers and designers are still in the phase of actually defining how multi-monitor support will behave from a user perspective.