XAPI HTTP PUT for VM exports

The idea is simple: allow XenServer to export a VM while pushing. Today, an export will create a HTTP handler ready to be GET for someone.

But in some case, it's not really convenient at all. Especially in various backup scenarios, as explained here: https://github.com/vatesfr/xo-web/issues/1201

We got a lot of pressure from users to allow that, so I'll be really glad that someone take a look I know - from last Xen Hackathon - that Jon Ludlam could do it on your side. I also know you got a lot of work to do, but this shouldn't be a too complicated feature to implement (somehow, you are already pushing VM with cross pool migration!)

It would also open a lot of other possibilities.