qemu_driver: Resolve Coverity RESOURCE_LEAK
qemu_driver: Resolve Coverity RESOURCE_LEAK

Coverity found that the 'buf' wasn't VIR_FREE'd at exit.

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