This patch adds support for the recent ipset iptables extension
to libvirt's nwfilter subsystem. Ipset allows to maintain 'sets'
of IP addresses, ports and other packet parameters and allows for
faster lookup (in the order of O(1) vs. O(n)) and rule evaluation
to achieve higher throughput than what can be achieved with
individual iptables rules.

On the command line iptables supports ipset using

iptables ... -m set --match-set <ipset name> <flags> -j ...

where 'ipset name' is the name of a previously created ipset and
flags is a comma-separated list of up to 6 flags. Flags use 'src' and 'dst'
for selecting IP addresses, ports etc. from the source or
destination part of a packet. So a concrete example may look like this:

iptables -A INPUT -m set --match-set test src,src -j ACCEPT

Since ipset management is quite complex, the idea was to leave ipset 
management outside of libvirt but still allow users to reference an ipset.
The user would have to make sure the ipset is available once the VM is
started so that the iptables rule(s) referencing the ipset can be created.

Using XML to describe an ipset in an nwfilter rule would then look as
follows:

  <rule action='accept' direction='in'>
    <all ipset='test' ipsetflags='src,src'/>
  </rule>

The two parameters on the command line are also the two distinct XML attributes
'ipset' and 'ipsetflags'.

FYI: Here is the man page for ipset:

https://ipset.netfilter.org/ipset.man.html

Regards,
    Stefan