commit a495365d09138bf0f07504cfe9b6ea2de858e18e
Author: Jim Fehlig <jfehlig@suse.com>
Date:   Fri Nov 18 14:54:38 2011 -0700

    Don't copy sexpr node value that is an empty string
    
    Xen4.1 initializes some unspecified sexpr config items to an empty
    string, unlike previous Xen versions that would leave the item unset.
    E.g. the kernel item for an HVM guest (non-direct kernel boot):
    
    Xen4.0 and earlier
      ...
      (image
        (hvm
          (kernel )
      ...
    
    Xen4.1
      ...
      (image
        (hvm
          (kernel '')
      ...
    
    The empty string for kernel causes some grief in subsequent parsing
    where existence of specified kernel is checked, e.g.
    
      if (!def->os.kernel)
      ...
    
    This patch solves the problem in sexpr_node_copy() by not copying
    a node containing an empty string.

Index: libvirt-0.9.7/src/util/sexpr.c
===================================================================
--- libvirt-0.9.7.orig/src/util/sexpr.c
+++ libvirt-0.9.7/src/util/sexpr.c
@@ -519,7 +519,7 @@ int sexpr_node_copy(const struct sexpr *
 {
     const char *val = sexpr_node(sexpr, node);
 
-    if (val) {
+    if (val && *val) {
         *dst = strdup(val);
         if (!(*dst))
             return -1;