Resolve valgrind error in virStorageBackendCreateQemuImgCmd()

Resolve valgrind error in virStorageBackendCreateQemuImgCmd()

Commit id '53d5967c' introduced the following:

TEST: storagevolxml2argvtest
      ..............                           14  OK
==25636== 358 (264 direct, 94 indirect) bytes in 1 blocks are definitely lost in loss record 67 of 75
==25636==    at 0x4A06B6F: calloc (vg_replace_malloc.c:593)
==25636==    by 0x4C95791: virAlloc (viralloc.c:124)
==25636==    by 0x4CA0BB4: virCommandNewArgs (vircommand.c:805)
==25636==    by 0x4CA0C88: virCommandNew (vircommand.c:789)
==25636==    by 0x408602: virStorageBackendCreateQemuImgCmd (storage_backend.c:849)
==25636==    by 0x405427: testCompareXMLToArgvHelper (storagevolxml2argvtest.c:61)
==25636==    by 0x4064DF: virtTestRun (testutils.c:158)
==25636==    by 0x40516F: mymain (storagevolxml2argvtest.c:195)
==25636==    by 0x406B1A: virtTestMain (testutils.c:722)
==25636==    by 0x37C1021A04: (below main) (libc-start.c:225)
==25636==
PASS: storagevolxml2argvtest