Clean up the Xen event version checks and make events work with the late...

...st 4.5 master branch.

The configure script defined two different variables, XENEVENT4* and XEN_MEMACCESS_VERSION, which made things a bit convoluted to keep track of. This PR combines the two into XEN_EVENTS_VERSION, which defines if Xen supports 4.1 style events, 4.2 or 4.5.

The PR also updates the code to work with the latest 4.5 (unstable) branch.