Detected by valgrind. Leak introduced in commit 5745dc12.

* src/qemu/qemu_command.c: fix memory leak on failure and successful path.

* How to reproduce?
% valgrind -v --leak-check=full ./qemuargv2xmltest

* Actual result:

==2196== 80 bytes in 1 blocks are definitely lost in loss record 3 of 4
==2196==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==2196==    by 0x39CF07F6E1: strdup (in /lib64/libc-2.12.so)
==2196==    by 0x419823: qemuParseRBDString (qemu_command.c:1657)
==2196==    by 0x4221ED: qemuParseCommandLine (qemu_command.c:5934)
==2196==    by 0x422AFB: qemuParseCommandLineString (qemu_command.c:7561)
==2196==    by 0x416864: testCompareXMLToArgvHelper (qemuargv2xmltest.c:48)
==2196==    by 0x417DB1: virtTestRun (testutils.c:141)
==2196==    by 0x415CAF: mymain (qemuargv2xmltest.c:175)
==2196==    by 0x4174A7: virtTestMain (testutils.c:696)
==2196==    by 0x39CF01ECDC: (below main) (in /lib64/libc-2.12.so)
==2196==
==2196== LEAK SUMMARY:
==2196==    definitely lost: 80 bytes in 1 blocks
Signed-off-by: NAlex Jia <ajia@redhat.com>