xen: maintain clock offset over save/restore

Hook into the device model to make sure that timekeeping's resume handler
is called.  This deals with our clocksource's non-monotonicity over the
save/restore.  Explicitly call clock_has_changed() to make sure that
all the timers get retriggered properly.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>