Reuse property-pre-set:netvm event for processing default_netvm change

Do not duplicate the check if changing to non-running netvm. But more
importantly, call the property-pre-set event to unset old netvm.
Otherwise setting the new one will fail (property-reset:netvm handler
assumes matching pre-(re)set handler detached old value already).
DO not use property-pre-reset:netvm here, because it can't know the new
value yet (pre-reset is used when changing non-default to default value,
so normally it can check what is the default value at the call time -
but here it wouldn't work because the default value is going to change).

Fixes QubesOS/qubes-issues#10125

Author: marmarek

qubes/app.py

@@ -1700,18 +1700,18 @@ def on_property_pre_set_default_netvm(
         self, event, name, newvalue, oldvalue=None
     ):
         # pylint: disable=unused-argument,invalid-name
-        if (
-            newvalue is not None
-            and oldvalue is not None
-            and oldvalue.is_running()
-            and not newvalue.is_running()
-            and self.domains.get_vms_connected_to(oldvalue)
-        ):
-            raise qubes.exc.QubesVMNotRunningError(
-                newvalue,
-                "Cannot change {!r} to domain that "
-                "is not running ({!r}).".format(name, newvalue.name),
-            )
+        for vm in self.domains:
+            if hasattr(vm, "netvm") and vm.property_is_default("netvm"):
+                # Use pre-set event instead of pre-reset, so it can get both
+                # old and new values, and perform necessary actions.
+                # Especially, it needs to detach old netvm.
+                vm.fire_event(
+                    "property-pre-set:netvm",
+                    pre_event=True,
+                    name="netvm",
+                    newvalue=newvalue,
+                    oldvalue=oldvalue,
+                )
 
     @qubes.events.handler("property-set:default_netvm")
     def on_property_set_default_netvm(