vmx: use xmlBufferDetach() when applicable

xmlBuffer->content was deprecated in libxml2 v2.13.0-33-gb34dc1e4
xmlBufferDetach(xmlBuffer) should be used instead

Signed-off-by: Jakub Palacky <jpalacky@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/vmx/vmx.c

@@ -794,7 +794,7 @@ virVMXConvertToUTF8(const char *encoding, const char *string)
         goto cleanup;
     }
 
-    result = (char *)g_steal_pointer(&utf8->content);
+    result = (char *)xmlBufferDetach(utf8);
 
  cleanup:
     xmlCharEncCloseFunc(handler);