VMM/Alias: renamed Alias.add_destination() to add_destinations().
Now it's possible to add one ore more destinations to the alias with a
single method call.
VMM/Handler: adjusted Handler.aliasAdd() to the API changes of the Alias
class. Also use get_gid from the Domain module to get the gid of a
domain. We don't need complete Domain object, only the gid.
Handler.getWarnings(): no longer return the __warnings list. Return a
copy instead and empty the Handler.__warnings list.