The ebuilds for app-emulation/virt-manager list dubs[X] as required when the gui use flag is activated. From my (short) investigation, dbus[X] only enables autolaunch of dbus when X is started. If this is the case, the dependency should be dbus regardless of the state of the X use flag. I tested an ebuild modified accordingly and can confirm that everything builds and works as intended on my system (x86-64/no-multilib) where the X flag is disabled globally, only Wayland is available (through sway), and dbus is built without the X flag.
