Consistent preload template gathering

- Easier copy-paste when code duplication is necessary;
- Only fires event if the qube should be really preloaded, which is more
  difficult to do considering the global feature. The prior behavior
  didn't error out, just a warning log;
- Add tests

For: QubesOS/qubes-issues#1512

Author: ben-grande

linux/aux-tools/preload-dispvm

@@ -12,39 +12,43 @@ import concurrent.futures
 import qubesadmin
 
 
-def get_max(qube):
-    return int(qube.features.get("preload-dispvm-max", 0) or 0)
+def get_preload_max(qube) -> int | None:
+    value = qube.features.get("preload-dispvm-max", None)
+    return int(value) if value else value
 
 
 async def main():
     app = qubesadmin.Qubes()
     domains = app.domains
     default_dispvm = getattr(app, "default_dispvm", None)
+    global_max = get_preload_max(domains["dom0"])
     appvms = [
         qube
         for qube in domains
-        if get_max(qube) > 0
-        and (
-            (
-                qube.klass == "AppVM"
-                and getattr(qube, "template_for_dispvms", False)
+        if (
+            qube.klass == "AppVM"
+            and getattr(qube, "template_for_dispvms", False)
+            and (
+                (qube != default_dispvm and get_preload_max(qube))
+                or (
+                    (qube == default_dispvm and global_max)
+                    or (global_max is None and get_preload_max(qube))
+                )
             )
-            or (qube.name == "dom0" and default_dispvm)
         )
     ]
     method = "admin.vm.CreateDisposable"
     loop = asyncio.get_running_loop()
     tasks = []
-    if "dom0" in appvms and default_dispvm in appvms:
-        appvms.remove(default_dispvm)
     with concurrent.futures.ThreadPoolExecutor() as executor:
         for qube in appvms:
-            maximum = get_max(qube)
-            msg = f"{qube}:{maximum}"
-            if qube.name == "dom0":
-                qube = default_dispvm
-                msg = "global:" + msg
-            print(msg)
+            if qube == default_dispvm and global_max is not None:
+                maximum = global_max
+                msg = f"global:{qube}:{maximum}"
+            else:
+                maximum = get_preload_max(qube)
+                msg = f"{qube}:{maximum}"
+            print(repr(msg))
             exec_args = qube.qubesd_call, qube.name, method, "preload-autostart"
             future = loop.run_in_executor(executor, *exec_args)
             tasks.append(future)