docs: correct and update network vlan example
docs: correct and update network vlan example

Somehow I put an example of a domain interface with a <vlan> element
into the network documentation.

This patch replaces that with an example of a network definition that
has a vlan element with trunk='yes', multiple tags, and even the new
nativeMode attribute. It also includes a <portgroup> that has a vlan
defined.

eskultety/libvirt,taget/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,agx/libvirt,taget/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jfehlig/libvirt,taget/libvirt,rlaager/libvirt,elmarco/libvirt,VenkatDatta/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,jardasgit/libvirt,libvirt/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,rlaager/libvirt,andreabolognani/libvirt,eskultety/libvirt,olafhering/libvirt,shugaoye/libvirt,crobinso/libvirt,rlaager/libvirt,jfehlig/libvirt,libvirt/libvirt,elmarco/libvirt,nertpinx/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,datto/libvirt,eskultety/libvirt,agx/libvirt,fabianfreyer/libvirt,rlaager/libvirt,agx/libvirt,agx/libvirt,nertpinx/libvirt,andreabolognani/libvirt,elmarco/libvirt,zippy2/libvirt,elmarco/libvirt,taget/libvirt,jfehlig/libvirt,crobinso/libvirt,eskultety/libvirt,shugaoye/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,shugaoye/libvirt,libvirt/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,agx/libvirt,taget/libvirt,datto/libvirt,andreabolognani/libvirt,libvirt/libvirt,shugaoye/libvirt,olafhering/libvirt,cbosdo/libvirt,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,zippy2/libvirt,datto/libvirt,cbosdo/libvirt,jardasgit/libvirt,elmarco/libvirt,rlaager/libvirt,cbosdo/libvirt,nertpinx/libvirt,nertpinx/libvirt,datto/libvirt,jfehlig/libvirt,cbosdo/libvirt,jardasgit/libvirt