network: fix crash when starting a network with no <pf> element

network: fix crash when starting a network with no <pf> element

Martin Kletzander pointed out in email that my commit 2a193f64
introduced a crash in networkCreateInterfacePool() during startup of
any network that doesn't have a <pf> subelement of its <forward>
element. He also supplied a patch.

 http://www.redhat.com/archives/libvir-list/2014-August/msg00655.html

I expanded on that patch by cleaning up now-extraneous checks in the
callers of networkCreateInterfacePool().

Fortunately the offending patch hasn't been in any release, and hasn't
been (to my knowledge) backported to any other branch.