GNOME Boxes is a very easy-to-use program from the GNOME Desktop that uses libvirt for managing virtual machines with the various forms of Linux virtualization or connecting to a remote system. Of course, virt-manager remains the dominant open-source Linux solution in this area, but GNOME Boxes continues to work out very nicely for strictly desktop users and other simple use-cases.
A new feature coming to GNOME Boxes via this year's Google Summer of Code is multiple monitor virtualization. Multiple monitor virtualization has been implemented already within other virtualization management solutions, but now it's coming to the GNOME stack by a student developer.
The goal is when connecting to a virtual machine with multiple display outputs, the primary display is the default but there will be a "monitor switcher" similar to a Linux desktop workspace switcher for handling the other outputs. Likewise, there's also plans for a preview mode of all the outputs.
It's still being figured out how the multi-monitor VM implementation will be for GNOME Boxes, but there's a status update via this blog post with the GSoC work being done by Adrien Plazas.