Fix memory leak in testGetCaps()

While running qemucaps2xmltest, it was found that valgrind pointed out
the following memory leaks:

==27045== 160 (112 direct, 48 indirect) bytes in 1 blocks are definitely lost in loss record 51 of 65
==27045==    at 0x4A0577B: calloc (vg_replace_malloc.c:593)
==27045==    by 0x4C6BACD: virAllocVar (viralloc.c:560)
==27045==    by 0x4CAF095: virObjectNew (virobject.c:193)
==27045==    by 0x421453: virQEMUCapsNew (qemu_capabilities.c:1805)
==27045==    by 0x41F04F: testQemuCapsXML (qemucaps2xmltest.c:72)
==27045==    by 0x41FFD1: virtTestRun (testutils.c:201)
==27045==    by 0x41EE7A: mymain (qemucaps2xmltest.c:203)
==27045==    by 0x42074D: virtTestMain (testutils.c:789)
==27045==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
==27045==
==27045== 160 (112 direct, 48 indirect) bytes in 1 blocks are definitely lost in loss record 52 of 65
==27045==    at 0x4A0577B: calloc (vg_replace_malloc.c:593)
==27045==    by 0x4C6BACD: virAllocVar (viralloc.c:560)
==27045==    by 0x4CAF095: virObjectNew (virobject.c:193)
==27045==    by 0x421453: virQEMUCapsNew (qemu_capabilities.c:1805)
==27045==    by 0x41F04F: testQemuCapsXML (qemucaps2xmltest.c:72)
==27045==    by 0x41FFD1: virtTestRun (testutils.c:201)
==27045==    by 0x41EEA3: mymain (qemucaps2xmltest.c:204)
==27045==    by 0x42074D: virtTestMain (testutils.c:789)
==27045==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)

tests/qemucaps2xmltest.c

@@ -125,6 +125,7 @@ testGetCaps(char *capsData, const testQemuData *data)
         goto error;
     }
 
+    virObjectUnref(qemuCaps);
     return caps;
 
  error: