vbox: Reset @ret after xmlFreeNode

In the error path, if we xmlFreeNode @ret, then the return ret;
a few lines later returns something that's already been free'd
and could be reused, so let's reinit it.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/vbox/vbox_snapshot_conf.c

@@ -352,6 +352,7 @@ virVBoxSnapshotConfCreateHardDiskNode(virVBoxSnapshotConfHardDiskPtr hardDisk)
     if (result < 0) {
         xmlUnlinkNode(ret);
         xmlFreeNode(ret);
+        ret = NULL;
     }
     VIR_FREE(uuid);
     return ret;