virsh: Don't traverse childless nodes in vshNodeIsSuperset

If both nodes do not have any children, we pass zero to
virBitmapAlloc which returns NULL. In turn we report OOM error
and return false (meaning nodes are different). This is not true.

tools/virsh.c

@@ -11556,6 +11556,9 @@ vshNodeIsSuperset(xmlNodePtr n1, xmlNodePtr n2)
     if (n1_child_size < n2_child_size)
         return false;
 
+    if (n1_child_size == 0 && n2_child_size == 0)
+        return true;
+
     if (!(bitmap = virBitmapAlloc(n1_child_size))) {
         virReportOOMError();
         return false;