We should ensure that the first expression in the permisions list
is nX, where X is the owning domain, and not rX or wX, as otherwise
we would be granting all other VMs read access to the key.
This is explained in more detail here:
http://wiki.xensource.com/xenwiki/XenBus
In practice the perms problem applied only to the qubes_netvm_external_ip key
that is exposed by each NetVM to corresponding Proxy VMs. Before this fix,
the key was readable by any VM in the system, which might not be desired in some
more advanced networking setups, such as with Tor Proxy VM.
59f71f634a