network: fix waiting for VM network uplink
The network-uplink-wait.sh script may be called before xen-netfront
module is even loaded (by udev). In that case, `get_qubes_managed_iface`
will fail to get the interface name and the wait will be skipped.
Fix this by loading xen-netfront module explicitly (do not try to
synchronize with udev, which is tricky not knowing the device
name).