Comment 6 for bug 1720887

IIRC Openstack adds two in most cases, like:
    <serial type='file'>
      <source path='/var/lib/nova/instances/7c0dcd78-d6b4-4575-a882-ee5d29c64fe0/console.log'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='file'>
      <source path='/var/lib/nova/instances/7c0dcd78-d6b4-4575-a882-ee5d29c64fe0/console.log'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>

Twice the same path btw, but also same alias.
I checked and that still means only 2 files per guest.
The default guest log like:
  /var/log/libvirt/qemu/x-test1.log
And the file redirected console like:
  /tmp/consoleIDX2.log

So under these conditions the assumption of ~4096 should work.
In detail I found that under this config there also are 2 pipes along the two logs.
So a single guest looks like:
l-wx------ 1 root root 64 Sep 25 20:48 22 -> /var/log/libvirt/qemu/x-test3.log
lr-x------ 1 root root 64 Sep 25 20:48 23 -> pipe:[5014140]
l-wx------ 1 root root 64 Sep 25 20:48 24 -> /tmp/consoleIDX3.log
lr-x------ 1 root root 64 Sep 25 20:48 25 -> pipe:[5014157]

With 5 guests I had 20 new FDs on the virtlogd service.
That said the limit should still be enough for 1024 guests.
Testing against those numbers now ...