x86/cpuid: Plug memory leak in cpudef_setfield()

To reproduce the leak, put two name options into the same [cpudef]
section of target-x86_64.conf.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
