I would recommend you to put your customizations at '/etc/multipath/conf.d/<somename>.conf' (you have to create the dir), so vdsm can keep multipath.conf up to date.

For details check the header of multipath.conf

Here is mine:

# The recommended way to add configuration for your storage is to add a

# drop-in configuration file in "/etc/multipath/conf.d/<mydevice>.conf".

# Settings in drop-in configuration files override settings in this

# file.

Of course you need to rebuild dracut.