VMM/Domain: added function get_gid() to the Domain module. We don't need to load all the domain related information from the database, when we need only the GID of a domain. For example in the Alias or Relocated classes.

# -*- coding: UTF-8 -*-
# Copyright (c) 2007 - 2010, Pascal Volk
# See COPYING for distribution information.

MISSING_ARGS = 1
UNKNOWN_COMMAND = 2
USER_INTERRUPT = 3