initialize backup_dir after mounting backup device (#587)

qubesmanager/backup_utils.py

@@ -147,6 +147,7 @@ def dev_combobox_activated(dialog, idx):
     dialog.prev_dev_idx = idx
     # Initialize path with root of mounted device
     dialog.dir_line_edit.setText(dialog.dev_mount_path)
+    dialog.backup_dir = dialog.dev_mount_path
     dialog.select_dir_page.emit(SIGNAL("completeChanged()"))