xen: Port Xen event channel driver from mini-os

Make required updates to run on u-boot. Strip functionality
not needed by U-boot.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>

include/xen.h

@@ -8,7 +8,7 @@
 /**
  * xen_init() - Xen initialization
  *
- * Map Xen memory pages.
+ * Map Xen memory pages, initialize event handler.
  */
 void xen_init(void);