d/libvirt-daemon-config-{network,nwfilter}.postinst: reload libvirtd before restart

Without this upgrades will trigger a warning level log entry like:
  Setting up libvirt-daemon-config-network (7.0.0-1ubuntu1~ppa1) ...
  Warning: The unit file, source configuration file or drop-ins of
  libvirtd.service changed on disk. Run 'systemctl daemon-reload' to
  reload units.

This is due to our restart of libvirtd.service, after unpacking
libvirt-daemon-system (owning libvirtd.service) but before (order implied by
the dependencies from libvirt-daemon-system -> libvirt-daemon-config-network)
configuring libvirt-daemon-system which otherwise would do this reload.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>