implement xen pv_time_ops to account steal time.

Cc: Jeremy Fitzhardinge <jeremy@goop.org>
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>