ia64/pv_ops: update Kconfig for paravirtualized guest and xen.

introduce CONFIG_PARAVIRT_GUEST, CONFIG_PARAVIRT for
paravirtualized guest.
introduce CONFIG_XEN, CONFIG_IA64_XEN_GUEST for xen.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>