migration: change has_error to contain errno values

We normally already have an errno value.  When not, abuse EIO.

Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/migration.c b/migration.c
index a682168..d5876a9 100644
--- a/migration.c
+++ b/migration.c
@@ -455,7 +455,7 @@
     } while (ret == -1 && (s->get_error(s)) == EINTR);
 
     if (ret == -1) {
-        qemu_file_set_error(s->file);
+        qemu_file_set_error(s->file, -s->get_error(s));
     }
 }