virconf: Report an error in when virConfSetValue() fails

Callers of virConfSetValue() don't report any error, they just
pass the error blindly. Therefore, report an error when
virConfSetValue() is about to fail.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/util/virconf.c

@@ -1329,6 +1329,9 @@ virConfSetValue(virConf *conf,
     virConfEntry *prev = NULL;
 
     if (value && value->type == VIR_CONF_STRING && value->str == NULL) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("expecting a value for value of type %s"),
+                       virConfTypeToString(VIR_CONF_STRING));
         virConfFreeValue(value);
         return -1;
     }