[IA64] set itv handoff as masked and enable reading irr[0-3]

[IA64] set itv handoff as masked and enable reading irr[0-3]

Set initial vcpu itv handoff state to mask the timer vector.
This seems to match hardware and makes logical sense from a
spurious interrupt perspective.  Enable vcpu_get_irr[0-3]
functions as they seem to work and have the proper backing.
This enables the check_sal_cache_flush() in arch/ia64/kernel.sal.c
to work unmodified, allowing us to remove the Xen changes from
the file (and thus the file from the sparse tree).

Signed-off-by: Alex Williamson <alex.williamson@hp.com>