The openvswitch-nova init.d script used on XenServer has been set to run at 96 in the boot order (i.e. very late). This is inconvenient, because it means that it runs after the firstboot scripts, so if you want to do an automated install of a compute VM, say, you can't rely on the Open vSwitch rules being ready.
It was moved to 96 from 10, after finding that the Open vSwitch itself wasn't initialized properly at this point. This is not a surprise, since network runs at 10, management-interface runs at 13, and loadbrsysctl is at 14. It looks like any time between 15 and 23 would be good (firstboot is at 24).
All init.d numbers are wrt XenServer dom0, which is derived from CentOS 5. XCP installations may vary, but I expect that they are similarly set up.