backup: open backup collection in offline mode

Otherwise it may try to manipulate live libvirt objects with the same
names (and probably different UUIDs, which would fail).

qubes/backup.py

@@ -1617,7 +1617,8 @@ class BackupRestore(object):
         """
         if self.header_data.version == 1:
             backup_app = qubes.core2migration.Core2Qubes(
-                os.path.join(self.backup_location, 'qubes.xml'))
+                os.path.join(self.backup_location, 'qubes.xml'),
+                offline_mode=True)
             return backup_app
         else:
             self._verify_hmac("qubes.xml.000", "qubes.xml.000.hmac")
@@ -1634,9 +1635,10 @@ class BackupRestore(object):
 
         if self.header_data.version in [2, 3]:
             backup_app = qubes.core2migration.Core2Qubes(
-                os.path.join(self.tmpdir, 'qubes.xml'))
+                os.path.join(self.tmpdir, 'qubes.xml'), offline_mode=True)
         else:
-            backup_app = qubes.Qubes(os.path.join(self.tmpdir, 'qubes.xml'))
+            backup_app = qubes.Qubes(os.path.join(self.tmpdir, 'qubes.xml'),
+                offline_mode=True)
         # Not needed anymore - all the data stored in backup_app
         os.unlink(os.path.join(self.tmpdir, 'qubes.xml'))
         return backup_app