ia64/pv_ops/xen: paravirtualize DO_SAVE_MIN for xen.

paravirtualize DO_SAVE_MIN in minstate.h for xen.

Signed-off-by: Yaozu (Eddie) Dong <eddie.dong@intel.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>