CVE-2016-5195 Dirty COW

The problem is, if you are hacked you can't trust your system anymore,
because the intruder can installed anything he likes.

I would start from scratch.

The migration from openvz to lxc can work but does not have to.
Ok how long would it take to backup my openvz CT's, convert them to get back my apps ?
This dependence on many factors. Nobody can tell.

Would you tell me what backup method do to advise : Snapshot? Suspend? stop? and what compression for a good convertion.
"stop" is in the migration case.
 
Ok perhaps a rethorical question but As I have the choice between Proxmox 4 and 5 when I get a new server on OVH which one do I choose?
 
You should always take a current version.