Add stateless_property decorator for properties exposed through mgmt API

Management API gives access only to qubes.property. And this is
actually a good thing, so instead of extending it to access also
builtins.property, add a simple decorator to define read-only, stateless
qubes.property.

QubesOS/qubes-issues#2622

qubes/__init__.py

@@ -407,6 +407,16 @@ class property(object):  # pylint: disable=redefined-builtin,invalid-name
         return bool(value)
 
 
+def stateless_property(func):
+    '''Decorator similar to :py:class:`builtins.property`, but for properties
+    exposed through management API (including qvm-prefs etc)'''
+    return property(func.__name__,
+        setter=property.forbidden,
+        saver=property.DontSave,
+        default=func,
+        doc=func.__doc__)
+
+
 class PropertyHolder(qubes.events.Emitter):
     '''Abstract class for holding :py:class:`qubes.property`
 

qubes/vm/mix/net.py

@@ -98,19 +98,19 @@ class NetVMMixin(qubes.events.Emitter):
     #
 
 
-    @property
+    @qubes.stateless_property
     def visible_ip(self):
         '''IP address of this domain as seen by the domain.'''
         return self.features.check_with_template('net/fake-ip', None) or \
             self.ip
 
-    @property
+    @qubes.stateless_property
     def visible_gateway(self):
         '''Default gateway of this domain as seen by the domain.'''
         return self.features.check_with_template('net/fake-gateway', None) or \
             self.netvm.gateway
 
-    @property
+    @qubes.stateless_property
     def visible_netmask(self):
         '''Netmask as seen by the domain.'''
         return self.features.check_with_template('net/fake-netmask', None) or \
@@ -133,7 +133,7 @@ class NetVMMixin(qubes.events.Emitter):
         # does not happen, because qid < 253, but may happen in the future.
         return '10.137.{}.{}'.format((vm.qid >> 8) & 7, vm.qid & 7)
 
-    @property
+    @qubes.stateless_property
     def gateway(self):
         '''Gateway for other domains that use this domain as netvm.'''
         return self.visible_ip if self.provides_network else None

qubes/vm/qubesvm.py

@@ -502,7 +502,7 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
 
     # VMM-related
 
-    @property
+    @qubes.stateless_property
     def xid(self):
         '''Xen ID.