Cleaning up a few minor nits with Xen 4.1.2 support and some typos.

Xen 4.1.2 specific:
* Suppress error on call to xc_set_access_required because apparently
Xen 4.1.2 always fails.

Non 4.1 specific:
* Fixing a compiler warning.

* Fixing a dbprint.

* Return VMI_SUCCESS for timeouts with no events.  This is consistent
with the examples from xenaccess example itself and the underlying
poll().

* event_example: Need to destroy() if driver completed it's init
otherwise, events are still bound on the domain and can't be reattached.

* Keep VMI_INIT_EVENTS from PARTIAL->COMPLETE.

* Restoring original access flags after clearing reg_event.  Fields
declared as "in_" should not be modified by the library.