qemu-doc: Rework the network options chapter to make "-net" less prominent

qemu-doc: Rework the network options chapter to make "-net" less prominent

"-net" is clearly a legacy option. Yet we still use it in almost all
examples in the qemu documentation, and many other spots in the network
chapter. We should make it less prominent that users are not lured into
using it so often anymore. So instead of starting the network chapter with
"-net nic" and documenting "-net <backend>" below "-netdev <backend>"
everywhere, all the "-net" related documentation is now moved to the end
of the chapter. The new "-nic" option is moved to the beginning of the
chapter instead, with a new example that should demonstrate how "-nic"
can be used to shortcut "-device" with "-netdev". The examples in this
chapter are changed to use the "-device" and "-netdev" options or
"-nic" instead of "-net nic -net <backend>".

While we're at it, also remove a legacy remark about very old Linux
distributions. Also remove the "[...]" from the examples in this chapter
since we are not using this ellipsis in any other examples in our docu-
mentation.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>