MSV: Don't use ArrayIndexOutOfBoundsException

See https://github.com/orbeon/msv/commit/e1af2dfaac7a9af5817fdc7542ede601a84b6959
Fix InstanceMirror after 8452756d29ab0f4724aa1c90b90dfc215a0a9a50

- InstanceMirror handles xxforms-replace
- add InstanceEvent to instance events for convenience
Fix xforms-insert dispatch after 8452756d29ab0f4724aa1c90b90dfc215a0a9a50