vt doc: Improve libvirt network section

An out-of-the-box configuration of SLES with KVM or Xen provides a
wicked-managed L2 bridge setup for VM connectivity. libvirt virtual
networks provide an alternate network setup for VM connectivity but
must be configured separately. Since the wicked-managed setup is the
default, move it's description before that of libvirt's virtual
networks. Also remove the confusing text around isolated vs forwarded
networks. Finally, add a warning about enabling forwarding in the VM
host when enabling a libvirt virtual network.

See https://bugzilla.suse.com/show_bug.cgi?id=1166570 for more details.