Fix a couple of issues with the VIF script

1. Python 2.4 doesn't have the json module, so use simplejson instead
2. Prefer XENBUS_TYPE over the command line argument for the vif script
3. Pass the VM's uuid to setup_vif_rules rather than the domid

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Add back an important synchronisation point that had fallen off

This was breaking suspend/resume/migrate on windows guests with
PV drivers.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>