libvirt_xml.network_xml: add forward tag as dict

Removed `fwd_mode` attribute because it's not flexible
when dealing with multiple attributes. It's safe to do
this since it's not used anywhere.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #1841 from Hao-Liu/fix_network

libvirt_xml.network_xml: add forward tag as dict