Allow network selection for VDI export

Right now, all VDI export are going through management interface. This is a real limitation for all XS backup vendors, because in general mgmt is not the fastest link. XenServer HTTP export handler should be able to listen on a specified network to allow faster VDI export speed.

Furthermore, this feature is already working for Xen Storage motion, because you can already select a migration network, so it shouldn't be too complicated to apply for exports.

Again, this is vital to provide a proper way to backup VM disks. Right now, alternatives are pretty weak or nasty (using xe remotely, SSH calls etc.). That's why adding this to XAPI would be the best option IMHO.