conf: split out virtio net driver formatting
conf: split out virtio net driver formatting

Instead of checking upfront if the <driver> element will be needed
in a big condition, just format all the attributes into a string
and output the <driver> element if the string is not empty.

rlaager/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,rlaager/libvirt,fabianfreyer/libvirt,agx/libvirt,nertpinx/libvirt,taget/libvirt,datto/libvirt,datto/libvirt,datto/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,eskultety/libvirt,elmarco/libvirt,andreabolognani/libvirt,agx/libvirt,fabianfreyer/libvirt,taget/libvirt,eskultety/libvirt,crobinso/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,libvirt/libvirt,andreabolognani/libvirt,jardasgit/libvirt,agx/libvirt,crobinso/libvirt,taget/libvirt,eskultety/libvirt,datto/libvirt,zippy2/libvirt,elmarco/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt,rlaager/libvirt,taget/libvirt,elmarco/libvirt,shugaoye/libvirt,zippy2/libvirt,datto/libvirt,agx/libvirt,eskultety/libvirt,jardasgit/libvirt,taget/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,shugaoye/libvirt,eskultety/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,elmarco/libvirt,rlaager/libvirt,jardasgit/libvirt,jardasgit/libvirt,shugaoye/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,rlaager/libvirt,VenkatDatta/libvirt,elmarco/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,agx/libvirt