[XEN] Replace direct common-code access of evtchn_upcall_mask

[XEN] Replace direct common-code access of evtchn_upcall_mask
with local_event_delivery_* accessors.
Notes:
 1. Still some (read-only, debug) use in keyhandler.c
 2. Still accesses through current->vcpu_info.
Both above may need to be compiled only for architectures
that use event channels.

Signed-off-by: Keir Fraser <keir@xensource.com>