xen: switch to using core atomics for synchronization

Now that the atomic macros are always genuinely atomic on x86, they can
be used for synchronization with Xen. A single core VM isn't too
unusual, but actual single core hardware is uncommon.

Replace an open-coding of evtchn_clear_port() with the inline.

Substantially inspired by work done by Julien Grall <julien@xen.org>,
2014-01-13 17:40:58.

Reviewed by: royger
MFC after: 1 week