conf: explicitly initialize 'cpumask' variable
conf: explicitly initialize 'cpumask' variable

Build with clang fails with:

  CC       conf/libvirt_conf_la-domain_conf.lo
  conf/domain_conf.c:13377:9: error: variable 'cpumask' is used
  uninitialized whenever 'if' condition is true
  [-Werror,-Wsometimes-uninitialized]
      if (!(tmp = virXMLPropString(node, "cpuset"))) {
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and many other similar errors regarding the 'cpuset' variable.

Fix by explicitly initializing it with NULL.

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