Resolve valgrind error

Resolves the following valgrind error from qemuxml2argvtest:

==20393== 5 bytes in 1 blocks are definitely lost in loss record 2 of 60
==20393==    at 0x4A0883C: malloc (vg_replace_malloc.c:270)
==20393==    by 0x38D690A167: __vasprintf_chk (in /usr/lib64/libc-2.16.so)
==20393==    by 0x4CB0D97: virVasprintf (stdio2.h:210)
==20393==    by 0x4CB0E53: virAsprintf (virutil.c:2017)
==20393==    by 0x428DC5: qemuAssignDeviceAliases (qemu_command.c:791)
==20393==    by 0x41DF93: testCompareXMLToArgvHelper (qemuxml2argvtest.c:151)
==20393==    by 0x41F53F: virtTestRun (testutils.c:157)
==20393==    by 0x41DA9B: mymain (qemuxml2argvtest.c:885)
==20393==    by 0x41FB7A: virtTestMain (testutils.c:719)
==20393==    by 0x38D6821A04: (below main) (in /usr/lib64/libc-2.16.so)
==20393==

From qemu_command.c/line 791:

    if (def->rng) {
        if (virAsprintf(&def->rng->info.alias, "rng%d", 0) < 0)
            goto no_memory;
    }

src/conf/domain_conf.c

@@ -13760,6 +13760,7 @@ virDomainRNGDefFree(virDomainRNGDefPtr def)
         break;
     }
 
+    virDomainDeviceInfoClear(&def->info);
     VIR_FREE(def);
 }