vm/adminvm: add default_dispvm property to AdminVM

This is useful to select default DispVM template for VMs started
directly by the user. This makes sense as long as AdminVM == GUIVM.

QubesOS/qubes-issues#2974

qubes/vm/adminvm.py

@@ -49,6 +49,12 @@ class AdminVM(qubes.vm.BaseVM):
         default='00000000-0000-0000-0000-000000000000',
         setter=qubes.property.forbidden)
 
+    default_dispvm = qubes.VMProperty('default_dispvm',
+        load_stage=4,
+        allow_none=True,
+        default=(lambda self: self.app.default_dispvm),
+        doc='Default VM to be used as Disposable VM for service calls.')
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)