Fix clang warnings in xen_events

warning: implicit conversion from enumeration
type 'compat_memaccess_t' to different enumeration type 'hvmmem_access_t'

warning: comparison of constant
'MEMACCESS_INVALID' (-1) with expression of type 'mem_access_t'
(aka 'hvmmem_access_t') is always false
Merge pull request #223 from tklengyel/cast_fix

Fix clang warnings in xen_events