Disable "Disposable template" checkbox if not applicable

fixes: QubesOS/qubes-issues#7217

qubesmanager/settings.py

@@ -897,6 +897,9 @@ def __init_advanced_tab__(self):
         self.dvm_template_checkbox.setChecked(
             getattr(self.vm, 'template_for_dispvms', False))
 
+        if not hasattr(self.vm, 'template_for_dispvms'):
+            self.dvm_template_checkbox.setEnabled(False)
+
         self.provides_network_checkbox.setChecked(
             getattr(self.vm, 'provides_network', False))
         if self.provides_network_checkbox.isChecked():

ui/settingsdlg.ui

@@ -752,7 +752,7 @@ border-width: 1px;</string>
               <item row="3" column="0">
                <widget class="QCheckBox" name="dvm_template_checkbox">
                 <property name="toolTip">
-                 <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;Allows using this qube as a template for disposables. Disposables will inherit the qube's state (configuration, installed programs, etc.), but their state will not persist between restarts. &lt;/p&gt;&lt;p&gt;Setting this option will cause this qube to be listed as an option in the &amp;quot;Default disposable template&amp;quot; list for all other qubes. &lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+                 <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;Allows using this qube as a template for disposables. Disposables will inherit the qube's state (configuration, installed programs, etc.), but their state will not persist between restarts. &lt;/p&gt;&lt;p&gt;Setting this option will cause this qube to be listed as an option in the &amp;quot;Default disposable template&amp;quot; list for all other qubes.&lt;/p&gt;&lt;p&gt;Only AppVMs and StandaloneVMs could be turned in to Disposable Templates&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
                 </property>
                 <property name="text">
                  <string>Disposable template</string>