move arch/x86/xen/events.c undedr drivers/xen to share codes
with x86 and ia64. And minor adjustment to compile.
ia64/xen also uses events.c
Signed-off-by: Yaozu (Eddie) Dong
Signed-off-by: Isaku Yamahata
Signed-off-by: Jeremy Fitzhardinge
Signed-off-by: Ingo Molnar
Signed-off-by: Thomas Gleixner