CA-146736: Fix event registration after pool_migrate_complete

After running pool_migrate_complete, make sure the xapi events thread
re-registers so as to watch events for the newly-arrived VM.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1953 from johnelse/ca-146736

CA-146736: Fix event registration after pool_migrate_complete