vfio/migration: Refactor vfio_save_state() return value

Currently, vfio_save_state() returns 1 regardless of whether there is
more data to send or not. This was done to prevent a fast changing VFIO
device from potentially blocking other devices from sending their data,
as qemu_savevm_state_iterate() serialized devices.

Now that qemu_savevm_state_iterate() no longer serializes devices, there
is no need for that.

Refactor vfio_save_state() to return 0 if more data is available and 1
if no more data is available.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240304105339.20713-3-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>

hw/vfio/migration.c

@@ -529,11 +529,7 @@ static int vfio_save_iterate(QEMUFile *f, void *opaque)
     trace_vfio_save_iterate(vbasedev->name, migration->precopy_init_size,
                             migration->precopy_dirty_size);
 
-    /*
-     * A VFIO device's pre-copy dirty_bytes is not guaranteed to reach zero.
-     * Return 1 so following handlers will not be potentially blocked.
-     */
-    return 1;
+    return !migration->precopy_init_size && !migration->precopy_dirty_size;
 }
 
 static int vfio_save_complete_precopy(QEMUFile *f, void *opaque)