vms: init graceful shutdown for GUI

- add python modules for qemu protocols: QMP (hardware interactions) and QGA (guest service interaction)
- refactor state directory: remove name from path (already contains url)
- add impure vm test for basic qmp interaction
- simplify existing vm persistance test (factor out shared code)
- integrate graceful shutdown into GUI

the GUI integration still needs to be improved later:
- add fallback in case system doesn't react to powerdown button
- shutdown GUI switch fails if VM hasn't been started yet, and then remains in a wrong position

nixosModules/clanCore/vm.nix

@@ -30,6 +30,13 @@ let
     # required for issuing shell commands via qga
     services.qemuGuest.enable = true;
 
+    # required to react to system_powerdown qmp command
+    # Some desktop managers like xfce override the poweroff signal and therefore
+    #   make it impossible to handle it via 'logind' diretly.
+    services.acpid.enable = true;
+    services.acpid.handlers.power.event = "button/power.*";
+    services.acpid.handlers.power.action = "poweroff";
+
     boot.initrd.systemd.enable = true;
 
     # currently needed for system.etc.overlay.enable