nwfilter: Check for filter presence before open connect during teardown

nwfilter: Check for filter presence before open connect during teardown

https://bugzilla.redhat.com/show_bug.cgi?id=1608275

Instantiation of an nwfilter binding is only allowed when
the net->filter is defined for the network; however, the
teardown of the binding does not make this check. This
leaves open the possibility that the teardown could be
called during guest shutdown/teardown in session mode
resulting in the following error being logged:

    error : nwfilterConnectOpen:383 : internal error: unexpected
    nwfilter URI path '/session', try nwfilter:///system

So before going through the teardown processing, let's
be sure the network had a filter and then attempt to
get a connection. For session mode it's not even possible
create an nwfilter binding.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>