migration: Fix migration state update issue

If live migration is very fast and can be completed in 1 second,
the dirty_sync_count of MigrationState will not be updated.
Then you will see "dirty sync count: 0" in qemu monitor even if
the actual dirty sync count is not 0.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr.David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>