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[libvirt] RFC: put domain's interfaces into distinct namespaces

Hi, all!

There is performance issue with network filters and broadcast ethernet traffic.
If L2 segment is large enough (several thousands of VMs) then there is a lot of
broadcast ARP traffic (about frames 100/s). As aresult on host with several hundreds
VMs (say 300) we have kernel thread eating 100% of CPUs just for checking this traffic
against firewall rules. The problem is if there are rules in ebtables POSTROUTING chain
(clean-traffic is example of such filter) then when every single broadcast frame turns into
300, one for every distinct bridge port and then each one of these 300 is checked against
300 / 2 rules average to find chain for that port. As a result we have 100 * 300 * 300 / 2
= 4.5 * 10^6 rules checks per second. Kernel does not spread this workload onto
different CPUs and anyway this is wasting CPUs!

The simple solution is to put rules that ACCEPT ARP traffic into POSTROUTING
itself before any port specific chains. But this will affect non-VM ports too 
and host itself. So can we instead make a distinct network namespace for every
VM and put tap there, next add the bridge into the namespace too so we can apply
ebtables rules there and insert tap into the bridge. Finally connect the bridges
in root namespace and VM namespace by veth pair. As result in the situation
described above each cloned frame will be cheched only againt rules for this
very VM. The regular TCP traffic will have same benefits. On the other hand we
need a bridge and veth pair for every VM and some CPU power to process this extra
traffic path.

The proposed approach also fixes the problem of slow libvirtd restarting with
network filters ([1], [2]) as it is rather difficult to mess network rules in
different network namespace, at least restarting/reloading firewalld won't
hurt such rules so we just don't need to reinstantiate rules at all.

[1] [RFC] Faster libvirtd restart with nwfilter rules
  which continues in

[2] [PATCH v2 0/2] nwfilter: don't reinstantiate rules if there is no need to


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