qemu: plug memory leak

qemu: plug memory leak

Leak introduced in commit c1bc3d89.
Detected by valgrind:

==18462== 1,100 bytes in 1 blocks are definitely lost in loss record 183 of 184
==18462==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==18462==    by 0x4A06167: realloc (vg_replace_malloc.c:525)
==18462==    by 0x4AADBB: virReallocN (memory.c:161)
==18462==    by 0x4A975E: virBufferGrow (buf.c:117)
==18462==    by 0x4A9D92: virBufferVasprintf (buf.c:290)
==18462==    by 0x4A9EF7: virBufferAsprintf (buf.c:263)
==18462==    by 0x429488: qemuBuildControllerDevStr (qemu_command.c:1993)
==18462==    by 0x42C4B6: qemuBuildCommandLine (qemu_command.c:3803)
==18462==    by 0x41A604: testCompareXMLToArgvHelper (qemuxml2argvtest.c:124)
==18462==    by 0x41BB81: virtTestRun (testutils.c:141)
==18462==    by 0x416DFF: mymain (qemuxml2argvtest.c:369)
==18462==    by 0x41B277: virtTestMain (testutils.c:696)
==18462==
==18462== LEAK SUMMARY:
==18462==    definitely lost: 1,100 bytes in 1 blocks
==18462==    indirectly lost: 0 bytes in 0 blocks

* src/qemu/qemu_command.c (qemuBuildCommandLine): Clean up on success.

Signed-off-by: Alex Jia <ajia@redhat.com>