error: add error_setg_file_open() helper

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/util/error.c b/util/error.c
index 519f6b6..53b0435 100644
--- a/util/error.c
+++ b/util/error.c
@@ -71,6 +71,11 @@
     *errp = err;
 }
 
+void error_setg_file_open(Error **errp, int os_errno, const char *filename)
+{
+    error_setg_errno(errp, os_errno, "Could not open '%s'", filename);
+}
+
 Error *error_copy(const Error *err)
 {
     Error *err_new;