nwfilter: spawn thread for reloading on firewalld trigger

nwfilter: spawn thread for reloading on firewalld trigger

When firewalld is restarted or has its rules reloaded, we trigger a
reload of the nwfilter driver. This is done directly in the main
event loop thread which is a bad idea.

In a previous commit we fixed a actual deadlock problem with the
virStateReload API, when triggered from SIGHUP:

commit 33c6eb9689eb51dfe31dd05b24b3b6b1c948c267
Author: Jim Fehlig <jfehlig@suse.com>
Date:   Thu Mar 8 15:04:48 2018 -0700

    libvirtd: fix potential deadlock when reloading

The same deadlock problem previously existed with the firewalld reload
trigger, however, today it is not quite so series. The QEMU driver uses
a private event thread for each VM, so the particular deadlock would
not occur. None the less during the time the filters are reloading all
use of the event loop is blocked, which prevents APIs being serviced.

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>