docs: fix network XML documentation

docs: fix network XML documentation

A few people have attempted to use the new forwarding modes with older
versions of libvirt. The docs where the modes are described have
always stated the minimum required libvirt version, but the examples
at the end didn't, which I believe is what has caused the confusion.

Similarly, the section on portgroups now has a version tag added at
the beginning.

I also noticed that there was no example of defining a <dns> hostname,
so I added one, as well as making the domain name example more
recognizable (by adding ".com" to the domain).