Fix Coverity errors in driver/xen_events.c

Coverity ID 1237221: Uninitialized scalar variable (UNINIT)11. uninit_use: Using uninitialized value out_access.
Coverity ID 1237223 (#1 of 1): Structurally dead code (UNREACHABLE)unreachable: This code cannot be reached: errdone:
Merge pull request #200 from tklengyel/coverity_fixes2

Fix Coverity errors in driver/xen_events.c