Drupal views: Allow civicrm users to be filtered by domain id

In a multisite arrangement, when listing out a set of civicrm users via a Drupal view it would be nice to be able to filter those users to just those that are members of a particular domain. This would also avoid the situation where pulling in Drupal date creates duplicate listings for civicrm users who exist on multiple domains.

Please see attached patch.

A bit of future work could involve writing a custom handler to add the option to set the filter to 'current domain', as opposed to a fixed value as is the case with this patch.