qubesvm: do not try to define libvirt object in offline mode

The idea is to not touch libvirt at all.

qubes/app.py

@@ -483,7 +483,8 @@ class VMCollection(object):
             raise qubes.exc.QubesVMNotHaltedError(vm)
         self.app.fire_event('domain-pre-delete', pre_event=True, vm=vm)
         try:
-            vm.libvirt_domain.undefine()
+            if vm.libvirt_domain:
+                vm.libvirt_domain.undefine()
             # pylint: disable=protected-access
             vm._libvirt_domain = None
         except libvirt.libvirtError as e:

qubes/vm/qubesvm.py

@@ -570,6 +570,9 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
         if self._libvirt_domain is not None:
             return self._libvirt_domain
 
+        if self.app.vmm.offline_mode:
+            return None
+
         # XXX _update_libvirt_domain?
         try:
             self._libvirt_domain = self.app.vmm.libvirt_conn.lookupByUUID(