tools/qvm-backup-restore: fix default list of VMs to restore

By default restore all of them, not only the ones named "[" or "]"
(which are invalid name).

Fixes QubesOS/qubes-issues#2924

qubesadmin/tools/qvm_backup_restore.py

@@ -84,7 +84,7 @@
 parser.add_argument('backup_location', action='store',
     help="Backup directory name, or command to pipe from")
 
-parser.add_argument('vms', nargs='*', action='store', default='[]',
+parser.add_argument('vms', nargs='*', action='store', default=[],
     help='Restore only those VMs')