qemu: Fix memory leak when building -cpu argument

qemu: Fix memory leak when building -cpu argument

Reported by Alex Jia:

==21503== 112 (32 direct, 80 indirect) bytes in 1 blocks are
definitely lost in loss record 37 of 40
==21503==    at 0x4A04A28: calloc (vg_replace_malloc.c:467)
==21503==    by 0x4A8991: virAlloc (memory.c:101)
==21503==    by 0x505A6C: x86DataCopy (cpu_x86.c:247)
==21503==    by 0x507B34: x86Compute (cpu_x86.c:1225)
==21503==    by 0x43103C: qemuBuildCommandLine (qemu_command.c:3561)
==21503==    by 0x41C9F7: testCompareXMLToArgvHelper
(qemuxml2argvtest.c:183)
==21503==    by 0x41E10D: virtTestRun (testutils.c:141)
==21503==    by 0x41B942: mymain (qemuxml2argvtest.c:705)
==21503==    by 0x41D7E7: virtTestMain (testutils.c:696)