Allow setting the VIF backend for a network

If, on a network, other_config:backend_vm is set to the UUID of a VM, this VM is
used for the VIF backends (netback) of any VIFs on the network.

This is useful for internal networks (only), since the key is set on a network
rather than on a PIF.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>