[IA64] vti save-restore: clean up arch_get/set_info_guest()

[IA64] vti save-restore: clean up arch_get/set_info_guest()

- Update comment in copy_rbs()
- Don't warn when rbs_size = 0 for cpu initialization case.
- Remove struct vcpu_guest_context_regs::rbs_nat member which isn't used.
  and add num_phys_stacked to struct vcpu_guest_context_regs.
  so far rbs_nat and rbs_rnat isn't, so it is allowed to change the offset
  of rbs_rnat.
- Add check when setting vRR[].
- Don't set vRR[] if val is zero.

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