Increase default file handle limits for virtlogd

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: Christian Ehrhardt <christian.ehrhardt@canonical.com>