If I'm on my own system running virt-manager, remotely connected via QEMU/KVM/ssh/openbsd-netcat to a server running KVM/libvirt/QEMU, and I run "systemd reboot/poweroff" on that server, my client socket to the server immediately closes. Makes sense, unless libvirt-guests is gracefully shutting down guests. In that case, it would be nice to delay shutting down the client socket until all the guests were shut down, so I could watch the vm shutdown process through the virtual console.
I think the problem is that when you trigger the host shutdown, systemd shuts off the ssh daemon fairly soon, which is what kills your virt-manager connection. Libvirtd is still running, since it needs to be running for libvirt-guests service to work. So I don't think this is anything that can be fixed on the libvirt side