There is a problem with having separate default action ("policy") and
rules because it isn't possible to set both of them atomically at the
same time.
To solve this problem, always have policy 'drop' (as a safe default),
but by default have a single rule with action 'accept'
FixesQubesOS/qubes-issues#2869
First part - handling firewall.xml and rules formatting.
Specification on https://qubes-os.org/doc/vm-interface/
TODO (for dom0):
- plug into QubesVM object
- expose rules in QubesDB (including reloading)
- drop old functions (vm.get_firewall_conf etc)
QubesOS/qubes-issues#1815