Add a few aliases for startup/shutdown

These aliases are widely used to think of these operations and save some typing
too. Even though there is some thought to make start/stop the default operation
name I don't think this should happen for 1.2, for now adding it as an alias is
fine.

Reviewed-by: iustinp

scripts/gnt-instance

@@ -940,6 +940,8 @@ commands = {
 
 aliases = {
   'activate_block_devs': 'activate-disks',
+  'start': 'startup',
+  'stop': 'shutdown',
   }
 
 if __name__ == '__main__':