Xen events: prevent initialization for PV domains and perform more sanity checks
Minor typo fix and adding comments
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 underlyin
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.
Bugfix - libvmi/events.c : Previously, the callback invoked by g_hash_table_foreach* modified the SS events hashtable (forbidden by glib API documentation and throwing an assert as a result). Fixed in vmi_shutdown_single_step by destroying the singlestep events hashtable (and recreating it to reproduce initial state). This is safe because driver_shutdown_single_step unconditionally disables singlestep on all VCPUs.
Bugfix - libvmi/events.c : during teardown, test each events hashtable for validity before attemping manipulation and g_hash_table_destroy