what works (tested by myself) is puting your module for automaticaly load in /etc/conf.d/modules
[picasso alois]# cat /etc/conf.d/modules
# Linux users can define a list of modules for a specific kernel version,
# a released kernel version, a main kernel version or all kernel versions.
# The most specific versioned variable will take precedence.
# FreeBSD users can only use the modules="foo bar" setting.
modules="virtio_net"
after reboot i can see:
[picasso alois]# lsmod | grep virti
virtio_net 61440 0
net_failover 20480 1 virtio_net