In order to disable ipv6, you will have to disable checkpoint/restore (suspend/resume/migrate functionality) as the modules vzrst and vzcpt have ipv6 as a dependancy.

1. Edit /etc/init.d/vz, find and comment out this line:
   

   CPT_MODULES="vzcpt vzrst"

2. Disable ipv6 module by creating /etc/modprobe.d/ipv6-off.conf
   

   alias net-pf-10 off
alias ipv6 off

3. Reboot.