agent: move reboot check from load to run in VMChangeActivity

This has some advantages:

* Dmidecode as external tool is called less often, only when really
  needed (init, just before reboot). Before, this happened on every
  agent run.
* Less debug log messages.
* Activity doesn't change just by loading it.
* Non-privileged users can show the activity now.

PL-131813

pkgs/fc/agent/fc/maintenance/activity/vm_change.py

@@ -177,12 +177,11 @@ def _update_reboot_needed(self):
         else:
             self.reboot_needed = None
 
-    def load(self):
+    def run(self):
         self._update_reboot_needed()
-
-    # Running an VMChangeActivity is not needed, so no run method.
-    # The request manager handles the reboot required by this activity.
+        self.returncode = 0
 
     def resume(self):
-        # There's nothing to do so we can safely "retry" this activity.
+        # run() just checks if the reboot is needed at the moment so we can safely
+        # retry this activity.
         self.run()