Resolve valgrind error

Resolve valgrind error

As a result of commit id '19c345f2', 'make -C tests valgrind' has the
following for qemuxml2argvtest:

==22482== 197 (80 direct, 117 indirect) bytes in 1 blocks are definitely lost in loss record 101 of 120
==22482==    at 0x4A06B6F: calloc (vg_replace_malloc.c:593)
==22482==    by 0x4C6F301: virAlloc (viralloc.c:124)
==22482==    by 0x4C840FC: virSaveLastError (virerror.c:308)
==22482==    by 0x431882: qemuBuildCommandLine (qemu_command.c:8204)
==22482==    by 0x41E8F0: testCompareXMLToArgvHelper (qemuxml2argvtest.c:155)
==22482==    by 0x41FE9F: virtTestRun (testutils.c:157)
==22482==    by 0x419DEB: mymain (qemuxml2argvtest.c:654)
==22482==    by 0x4204DA: virtTestMain (testutils.c:719)
==22482==    by 0x39D0821A04: (below main) (libc-start.c:225)
==22482==