Server side of firewall support for XEN containers.

This differs from the current firewall support, which assumes a single
firewall for an entire experiment, hosted on a dedicated physical
node. At some point, it would be better to host the dedicated firewall
inside a XEN container, but that is a project for another day (year).

Instead, I added two sets of firewall rules to the default_firewall_rules
table, one for dom0 and another for domU. These follow the current
style setup of open,basic,closed, while elabinelab is ignored since it
does not make sense for this yet.

These two rules sets are independent, the dom0 rules can be applied to
the physical host, and domU rules can be applied to specific
containers.

My goal is that all shared nodes will get the dom0 closed rules (ssh
from local boss only) to avoid the ssh attacks that all of the racks
are seeing.

DomU rules can be applied on a per-container (node) basis. As
mentioned above this is quite different, and needed minor additions to
the virt_nodes table to allow it.