Add a safety valve to the HVM RTC model for big time jumps

Add a safety valve to the HVM RTC model for big time jumps

If xen's time leaps forward by a large amount, the RTC will try to
model a tick for every second that it thinks has passed.  This can
livelock a CPU with a series of timer requests each of which fires
immediately and requests the next one.

This patch treats a delay of more than a day between ticks as a
special case, abandoning the attempt to catch up.  That should be good
enough to avoid livelock but doesn't fix the underlying time problem.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>