I noticed a problem getting a Windows 7 guest to suspend when closing the Gnome Boxes window for it. It seems to be doing a hard power-off each time I did so.
To troubleshoot I did the following
sudo apt install -y virt-manager
virsh managedsave win7-ultimat
That allowed for manual suspend of the win7 guest. When I resumed the box, it was running right where I left it, no reboot.
Now when I go to close the Gnome Boxes window, the Windows 7 guest is suspending properly.