Increase default file handle limits for virtlogd

The initial assumption was ~2 files per guest, but some common setups
like Openstack drive up to 4 files per guest.

E.g. on Arm where the following XML leads to 4 file handles:
    <serial type='file'>
      <source path='/var/lib/nova/instances/7c0dcd78-.../console.log'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='file'>
      <source path='/var/lib/nova/instances/7c0dcd78-.../console.log'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>

With that in mind and the target to support 4k guests by default we
should raise the limit to 16k.
Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>

src/logging/virtlogd.service.in

@@ -15,9 +15,11 @@ ExecReload=/bin/kill -USR1 $MAINPID
 OOMScoreAdjust=-900
 # Need to have at least one file open per guest (eg QEMU
 # stdio log), but might be more (eg serial console logs)
+# A common case is OpenStack which often has up to 4 file
+# handles per guest.
 # libvirtd.service written to expect 4096 guests, so if we
-# guess at 2 log files per guest here (stdio + 1 serial):
-LimitNOFILE=8192
+# guess at 4 files per guest here that is 16k:
+LimitNOFILE=16384
 
 [Install]
 Also=virtlogd.socket