def remove_security_group(nova, vm, security_group):
    """
    Removes a security group from an existing VM
    :param nova: the nova client
    :param vm: the OpenStack server object (VM) to alter
    :param security_group: the SNAPS SecurityGroup domain object to add
    """
    nova.servers.remove_security_group(str(vm.id), security_group.name)
