xenserver: use ovs-vsctl for VIF VLANs instead of separate state files

xenserver: use ovs-vsctl for VIF VLANs instead of separate state files

ovs-vsctl did not exist when this code was originally written, but it
provides exactly what is needed to get rid of those separate state
files.

The vif hotplug script diff is against the xs5.7 branch but I think is
applicable to master and/or citrix with just context changes.

I was thinking of using ovs-vsctl exclusively for configuration
modifications from the vif hotplug script but that would need a
mechanism to pass the additional vif details to ovs-vsctl add-port as
well as perhaps making the bridge optional to del-port. The other option
would be to use the --no-reload option and split the config mods into
two parts, but I don't like that idea much.