storagevolxml2argvtest: Report better error messages on test failure

If the creation of the commandline failed, libvirt always reported "out
of memory" from the virCommandToString function rather than the proper
error that happened in virStorageBackendCreateQemuImgCmd. Error out
earlier.

tests/storagevolxml2argvtest.c

@@ -60,9 +60,7 @@ testCompareXMLToArgvFiles(bool shouldFail,
 
     cmd = virStorageBackendCreateQemuImgCmd(conn, &poolobj, vol, inputvol,
                                             flags, create_tool, imgformat);
-
-    actualCmdline = virCommandToString(cmd);
-    if (!actualCmdline) {
+    if (!cmd) {
         if (shouldFail) {
             virResetLastError();
             ret = 0;
@@ -70,6 +68,9 @@ testCompareXMLToArgvFiles(bool shouldFail,
         goto cleanup;
     }
 
+    if (!(actualCmdline = virCommandToString(cmd)))
+        goto cleanup;
+
     len = virtTestLoadFile(cmdline, &expectedCmdline);
     if (len < 0)
         goto cleanup;