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.
.. Substitutions in section 1
.. |account.disk_usage| replace:: `account.disk_usage`
.. _account.disk_usage: vmm.cfg.5#account-disk-usage
.. |domain.force_deletion| replace:: `domain.force_deletion`
.. _domain.force_deletion: vmm.cfg.5#domain-force-deletion
.. |misc.transport| replace:: `misc.transport`
.. _misc.transport: vmm.cfg.5#misc-transport
.. #EOF