'Dynamically limits hosts to one instance type
Return False if host has any instance types other than the requested
type. Return True if all instance types match or if host is empty.'

def host_passes(self, host_state, spec_obj):

    instance_type = spec_obj.flavor
    instance_type_id = instance_type.id
    other_types_on_host = utils.other_types_on_host(host_state, instance_type_id)
    return (not other_types_on_host)
