Fix typo in variable name

Fix typo in variable name

that broke the build like:

  CC     libvirt_conf_la-domain_conf.lo
  conf/domain_conf.c: In function 'virDomainVcpuPinAdd':
  conf/domain_conf.c:11920:29: error: 'vpcupin' undeclared (first use in this function)
  conf/domain_conf.c:11920:29: note: each undeclared identifier is reported only once for each function it appears in
  make[3]: *** [libvirt_conf_la-domain_conf.lo] Error 1