[IA64] paravirtualize itc and support save/restore.

[IA64] paravirtualize itc and support save/restore.

ia64 linux 2.6.18 only use ar.itc for local ticks so that
ar.itc didn't need paravirtualization and it can be work arounded
when save/restore.
However recent ia64 linux uses ar.itc for sched_clock() and
CONFIG_VIRT_CPU_ACCOUNTING and other issues. So ar.itc needs
paravirtualization. Although Most part is done in guest OS,
save/restore needs hypervisor support.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>