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.