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>

arch_init.c

@@ -263,7 +263,7 @@ int ram_save_live(Monitor *mon, QEMUFile *f, int stage, void *opaque)
     }
 
     if (cpu_physical_sync_dirty_bitmap(0, TARGET_PHYS_ADDR_MAX) != 0) {
-        qemu_file_set_error(f);
+        qemu_file_set_error(f, -EINVAL);
         return 0;
     }