cpuCompare: Fix crash on unexpected CPU XML
cpuCompare: Fix crash on unexpected CPU XML

When comparing a CPU without <model> element, such as

    <cpu>
        <topology sockets='1' cores='1' threads='1'/>
    </cpu>

libvirt would happily crash without warning.

elmarco/libvirt,jeckersb/libvirt,amery/libvirt-vserver,dumbbell/libvirt,warewolf/libvirt,amery/libvirt-vserver,datto/libvirt,datto/libvirt,VenkatDatta/libvirt,bjzhang/libvirt,elmarco/libvirt,agx/libvirt,libvirt/libvirt,rmarwaha/libvirt1,foomango/libvirt,shugaoye/libvirt,elmarco/libvirt,rbu/libvirt,kantai/libvirt-vfork,eskultety/libvirt,rmarwaha/libvirt1,usc-isi/libvirt,rbu/libvirt,VenkatDatta/libvirt,cbosdo/libvirt,jardasgit/libvirt,siboulet/libvirt-openvz,trainstack/libvirt,datto/libvirt,novel/fbsd-libvirt,amery/libvirt-vserver,dumbbell/libvirt,agx/libvirt,wiedi/libvirt,soulxu/libvirt-xuhj,rmarwaha/libvirt1,fabianfreyer/libvirt,emaste/libvirt,shugaoye/libvirt,soulxu/libvirt-xuhj,shugaoye/libvirt,eskultety/libvirt,rlaager/libvirt,elmarco/libvirt,novel/fbsd-libvirt,dumbbell/libvirt,olafhering/libvirt,shugaoye/libvirt,novel/fbsd-libvirt,datto/libvirt,novel/fbsd-libvirt,nertpinx/libvirt,iam-TJ/libvirt,cbosdo/libvirt,crobinso/libvirt,iam-TJ/libvirt,libvirt/libvirt,rmarwaha/libvirt1,crobinso/libvirt,usc-isi/libvirt,jardasgit/libvirt,jfehlig/libvirt,taget/libvirt,soulxu/libvirt-xuhj,zhlcindy/libvirt-1.1.4-maintain,trainstack/libvirt,agx/libvirt,jeckersb/libvirt,danwent/libvirt-ovs,wiedi/libvirt,usc-isi/libvirt,jeckersb/libvirt,nertpinx/libvirt,taget/libvirt,trainstack/libvirt,shugaoye/libvirt,iam-TJ/libvirt,taget/libvirt,fabianfreyer/libvirt,rlaager/libvirt,andreabolognani/libvirt,bjzhang/libvirt,amery/libvirt-vserver,jfehlig/libvirt,iam-TJ/libvirt,zippy2/libvirt,jardasgit/libvirt,novel/fbsd-libvirt,cbosdo/libvirt,jardasgit/libvirt,wiedi/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,emaste/libvirt,danwent/libvirt-ovs,warewolf/libvirt,nertpinx/libvirt,kantai/libvirt-vfork,eskultety/libvirt,datto/libvirt,soulxu/libvirt-xuhj,VenkatDatta/libvirt,trainstack/libvirt,kantai/libvirt-vfork,jeckersb/libvirt,agx/libvirt,iam-TJ/libvirt,zhlcindy/libvirt-1.1.4-maintain,libvirt/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,trainstack/libvirt,elmarco/libvirt,nertpinx/libvirt,warewolf/libvirt,leilihh/libvirt,rlaager/libvirt,warewolf/libvirt,danwent/libvirt-ovs,siboulet/libvirt-openvz,rlaager/libvirt,trainstack/libvirt,andreabolognani/libvirt,emaste/libvirt,leilihh/libvirt,eskultety/libvirt,bjzhang/libvirt,wiedi/libvirt,rmarwaha/libvirt,leilihh/libvirt,rmarwaha/libvirt1,soulxu/libvirt-xuhj,warewolf/libvirt,emaste/libvirt,rbu/libvirt,dumbbell/libvirt,foomango/libvirt,iam-TJ/libvirt,jeckersb/libvirt,rmarwaha/libvirt,novel/fbsd-libvirt,danwent/libvirt-ovs,sshah-solarflare/Libvirt-PCI-passthrough-,bjzhang/libvirt,siboulet/libvirt-openvz,cbosdo/libvirt,warewolf/libvirt,wiedi/libvirt,rmarwaha/libvirt,rbu/libvirt,siboulet/libvirt-openvz,foomango/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,siboulet/libvirt-openvz,bjzhang/libvirt,zippy2/libvirt,rmarwaha/libvirt,zhlcindy/libvirt-1.1.4-maintain,rmarwaha/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,usc-isi/libvirt,wiedi/libvirt,kantai/libvirt-vfork,novel/fbsd-libvirt,emaste/libvirt,andreabolognani/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,jeckersb/libvirt,usc-isi/libvirt,cbosdo/libvirt,novel/fbsd-libvirt,zippy2/libvirt,andreabolognani/libvirt,jeckersb/libvirt,taget/libvirt,rbu/libvirt,rmarwaha/libvirt1,jfehlig/libvirt,foomango/libvirt,kantai/libvirt-vfork,jfehlig/libvirt,emaste/libvirt,leilihh/libvirt,trainstack/libvirt,jardasgit/libvirt,emaste/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,wiedi/libvirt,rlaager/libvirt,iam-TJ/libvirt,dumbbell/libvirt,amery/libvirt-vserver,crobinso/libvirt,leilihh/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,rmarwaha/libvirt,warewolf/libvirt,dumbbell/libvirt,leilihh/libvirt,zippy2/libvirt,agx/libvirt,foomango/libvirt,eskultety/libvirt,novel/fbsd-libvirt,taget/libvirt,olafhering/libvirt,danwent/libvirt-ovs,crobinso/libvirt,libvirt/libvirt