Fixed default install_system state for Create New VM

fixes QubesOS/qubes-issues#5395

ui/newappvmdlg.ui

@@ -210,6 +210,9 @@
        </item>
        <item row="2" column="0" colspan="2">
         <widget class="QCheckBox" name="install_system">
+         <property name="enabled">
+          <bool>false</bool>
+         </property>
          <property name="text">
           <string>install system from device (also available from settings)</string>
          </property>