qubes: implement offline mode

Apparently the most important (the only?) property required in offline
mode is "is_running". So let's patch it to return False and make sure
any other libvirt usage would result in failure.

Or maybe better simply returh False in vm.is_running, when libvirt
connection fails? But then it would not be possible to use offline mode
and have (some, probably unrelated) libvirtd running at the same time.

Fixes QubesOS/qubes-issues#2008

qubes/app.py

@@ -126,25 +126,17 @@ class VirConnectWrapper(object):
 class VMMConnection(object):
     '''Connection to Virtual Machine Manager (libvirt)'''
 
-    def __init__(self):
+    def __init__(self, offline_mode=False):
         self._libvirt_conn = None
         self._xs = None
         self._xc = None
-        self._offline_mode = False
+        self._offline_mode = offline_mode
 
     @property
     def offline_mode(self):
         '''Check or enable offline mode (do not actually connect to vmm)'''
         return self._offline_mode
 
-    @offline_mode.setter
-    def offline_mode(self, value):
-        if value and self._libvirt_conn is not None:
-            raise qubes.exc.QubesException(
-                'Cannot change offline mode while already connected')
-
-        self._offline_mode = value
-
     def _libvirt_error_handler(self, ctx, error):
         pass
 
@@ -563,7 +555,7 @@ class Qubes(qubes.PropertyHolder):
         default=True,
         doc='check for updates inside qubes')
 
-    def __init__(self, store=None, load=True, **kwargs):
+    def __init__(self, store=None, load=True, offline_mode=False, **kwargs):
         #: logger instance for logging global messages
         self.log = logging.getLogger('app')
 
@@ -579,7 +571,7 @@ class Qubes(qubes.PropertyHolder):
         self.pools = {}
 
         #: Connection to VMM
-        self.vmm = VMMConnection()
+        self.vmm = VMMConnection(offline_mode=offline_mode)
 
         #: Information about host system
         self.host = QubesHost(self)

qubes/vm/qubesvm.py

@@ -1324,6 +1324,9 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
         :rtype: bool
         '''
 
+        if self.app.vmm.offline_mode:
+            return False
+
         # TODO context manager #1693
         return self.libvirt_domain and self.libvirt_domain.isActive()