[libvirt] network: allow tftp port if tftp is defined

[apevec at gmail.com]

From: Alan Pevec <apevec at redhat.com>

Libvirt managed virtual network can provide TFTP service,
in which case port 69/udp needs to be opened.

 1/2 bridge_driver.c: fix file description
 2/2 network: allow tftp port if tftp is defined